iTunes SD can be equal to DVD resolution, but it depends on the title. A DVD typically has a 300,000 px resolution (vertical*horizontal resolution), while iTunes SD offerings have varying resolutions between 100.000 px and 400.000 px (there are actually SD shows on iTunes that look better than DVD, e.g. LOST). You really have to watch the preview and use the screen grabber tool to measure if you're not sure of the size on iTunes (cmd-shift-4).
I just checked "The Blue Planet" on iTunes and it seems to be in 640x360 or about 200.000 px, which is not as high as the DVD resolution. So the DVD picture is probably richer but from what I can see the iTunes version is pretty and perfectly watchable.
Other criteria would be:
- The data rate. DVDs usually have 4-5 MBit/s in MPEG2. iTunes has 1.5-2 MBit/s in MPEG 4 which equals 3-5 MBit/s of the older MPEG2 codec. From my watching experience, iTunes is slightly more likely to show you artefacts than DVDs, but I have a suspicion that this depends more on how much care the producers put into converting the material than the actual data rate. No certain conclusions can be made for your show.
- This show has a very impressive soundtrack, and iTunes usually doesn't offer sourround sound.
- iTunes is only half the price of the DVD.
- according to Wikipedia the DVD comes with a bunch of bonus material. You may like to research how significant they are.
In the end you should opt for the DVD if you want the better quality and for iTunes if you like the savings and convenience of iTunes. For future purchases: Always check the preview on iTunes.
Well, I can't speak to the quality of specific titles, but there are other reasons to buy physical media: movies and TV shows come with DRM. Apple's music offerings are now DRM-free, but they are still compressed, and I rip most of my stuff to Apple Lossless.
DVDs have a copy protection and it's either illegal or a breach of contract to copy them. It's also quite a bit of work.
If you buy the DVDs, they are yours to bring over to a friend's house, for example. No such luck with downloads.
Not entirely true. You can bring iTunes downloads on your notebook, iPhone or iPod (with video cable) or on any type of media like a burnt DVD-Rom or USB stick. If you don't bring your own device all you need to remember is putting in your password into your friend's iTunes before you watch the movie and delete it after you've watched it. This is perfectly doable, and legal/allowed under Apple's terms.