Confused yet?
My subjective impression is that iTunes has increasingly offered movies with the option of an HD version (720p). Objectively I can quantify this, as in counting I can report that just in Dramas they offer 221 titles. In total they still do not offer what is available elsewhere, but the HD segment of their library is growing fairly fast.
Unfortunately strangely so. Look at upcoming releases, for instance, and you'll discover many titles offered in SD or HD. If perhaps an audience for every movie, some of these titles will safely never win an Academy Award; others that actually might sometimes are offered in SD only. Whether Apple, the studios, or someone else, games are being played. It is disconcerting, but true, that the new title just released only in SD for $14.99 will eventually sell for less, perhaps rather soon if not popular. The standard for anything not brand new is $9.99. That might be expected, but more maddening when discovering the SD title purchased for $14.99 appears several months later in HD for $19.99. If liking the film, then the higher price for HD would have been worth it, but that supposedly wasn't an option. Only now it is, after you've already paid $14.99 for SD. One might wait. Which may work, or possibly find that seemingly iTunes never intends offering an HD version.
This guessing game extends into that discounted. Many older, and sometimes excellent, movies are discounted to $4.99, either permanently or only temporarily. There are many older titles offered at $5.99. HD versions will be discounted as well, but not as many offered, and not in the same fashion. Some newer HD titles are seemingly offered initially at $17.99 instead of the more normal $19.99. Some older and, presumably, less popular can be found at well less than $10. 'Zoolander' was recently temporarily offered at $9.99 in HD.
Anyone preferring HD may be nonplused to find their favorite movie offered only in SD, but then a brief mention that it can be had in HD via Apple TV? What? That would have been a better avenue when one could actually buy a movie from ATV.
One way of getting a quick idea of resolution in iTunes is when stopping a title. This used to be a one-stop process: close the movie and one was back to the list of movies. More recently doing the same leaves the movie, etc., in a pop-up window, which must itself be closed, so two steps. But other than puzzling, this can be revealing. The size of the pop-up reveals the media's resolution. Older TV programs and other media presented in a 4:3 aspect ratio will be in a pop-up of the same dimension, but smaller than when at full screen. Please also note I am referencing this from a 13" MacBook, so one's experience may vary. Widescreen movies are of course more rectangular. SD versions will reveal a small pop-up about half the width of this 13" screen. HD versions are appreciably larger, reaching from edge to edge. How tall they are depends on the aspect ratio filmed in, but in width always side to side for HD. For SD, some movie pop-ups appear larger or smaller, depending presumably on varying degrees of resolution.
Then the issue of quality. Subjectively, it seems to be improving, or at least with far fewer of the egregious errors previously present. Meaning that even standard SD versions appear as their DVD cousins. There remains some variance. The HD version of 'Zoolander' is okay but obviously not the clarity of other HD offerings. This could be the film itself, in how filmed, or perhaps the transfer. Someone that knows more about this than myself tells me that even in BlueRay all is not equal, some movies transferred from film to a digital medium with more care and better results. On the iTunes front, however, I can tell you that the SD version of 'Sleepy Hollow' from iTunes was decidedly inferior to the same thing on DVD. Whether iTunes has rectified such an oversight I have no idea, but would caution you against buying 'Thunderbolt and Lightfoot' from them. Only instance I have seen, but the quality of this SD offering was okay, BUT it was delivered in the wrong aspect ratio. I mean seriously wrong, as in everything was stretched out. The entire movie that way. Apple easily refunded the price when informed of this, and I deleted it from the drive on my own accord. But some months later, since this film is expensive to purchase on DVD, I tried iTunes again. With the same exact result!
This brings us to the question of what one is going to be happiest with. Sometimes I wonder if I shouldn't just stick with HD, but a good movie in SD at $4.99 is often tempting. When it comes to HD, I also wonder why I am even settling for the iTunes version of it at 720p when the BlueRay version at 1080p can be had at the same price.
Maybe less. It pays to shop around. Anymore there is less rhyme and reason to this, and one might rent 'A Christmas Carol' from iTunes for $3.99, or rent the same thing from Amazon Video on Demand for .99¢ (it is now back up to $3.99 on VOD). One will discover that releases and prices on iTunes and VOD mirror one another. But on any given occasion one or the other may offer the same title for several dollars less. Then also, if one is willing to put up with a disc, then one should check beyond iTunes to what may be available on DVD or BlueRay. In example, some BlueRay titles (being twice the quality of Apple's HD) can at times be offered at the same approximate price as what Apple asks for an SD version.
Speaking of Amazon VOD, this service is a surprisingly good competitor to iTunes. In fact, with several advantages. For one, everything is instant: one can wait for a rental movie to download (or at least buffer sufficiently, particularly with a slow connection), or watch it now streaming from VOD. Then also, since this media is retained on Amazon's servers, one does not have to concern themselves with their own in managing and storing media. They also offer the option of downloading media, but since this has been historically a PC only option, I cannot speak to it. If allowing Amazon to hold your media, as it can be rented or purchased, then there can be downsides as well. If working quite easily and well, this service is dependent on your broadband connection. If without one, traveling, etc., you will not be watching anything. If, God forbid, Amazon ever went belly up then presumably say goodbye to all your movies stored with them. That possibility aside, what one will experience more often is how good your internet provider is. One advantage with iTunes, if potentially slower, is that once you've downloaded something you've really got it, seemingly in perpetuity. Assuming one's computer works, then, whether rented or bought, any movie should play fine from start to finish. However, since presently with a pitiful 1.5Mps (more like 1.3Mps max) connection, I can report that the standard SD movie from iTunes will download in about 3 hours. But it is possible to use VOD on even such a connection. They use a good algorithm which not only returns a good quality picture, but will adjust it as needed to suit the connection. If something like mine (cough), one may witness a crisp picture revert to something less, depending on the vagaries of the incoming signal. Generally quite good, although anyone else considering this might consider 1.5Mps more of a bare minimum. If, at times, the signal drops low enough then VOD will pause the film while it buffers. Even in my case, this is seldom. Presumably anyone in a city with appreciably better broadband would never notice this. But even SD offerings can look near iTunes HD with some movies, and generally good and acceptable otherwise. VOD also offers HD versions of some titles, but this requires something other than a computer, such as a newer internet HDTV.
Then there are other providers, such as Vudu, which in their case offer streaming 1080p (or nearly so), but in that case a connection of more like 5Mps required. It can all seem a bit much, all the more as everything is changing so fast. SD titles I purchased from iTunes, and thought to favor forever, may in time get replaced with HD versions from someone. Or maybe not. Particularly with some older movies, there are no HD versions available, seemingly anywhere, and may never be. But sometimes older movies are finally released in an HD version. Which makes all the more sense as even those only available in SD were captured on film which as master is better than even digitally downscaled BlueRay.
Never before as many options in this, all possibly different tomorrow.