efoto said:
The 2.8 isn't that much heavier than the 4 in my opinion, but the 2.8 IS adds in on without hold for sure.
I just went and looked up the respective weights...
f/4 = 1.56lbs
f/2.8 = 2.8lbs (+1.2lbs)
f/2.8 IS = 3.2lbs without tripod collar (+0.4lbs)
For comparison, The OD on the 2.8 is larger than the 4 though, if that matters.
The f/4 takes a 67mm filter, whereas the f/2.8 takes a 77mm filter.
Setting aside respective costs for polarizer filters and the like, I don't think that the difference between 67mm vs 77mm will make too much of a difference when it comes to camera stowage - - most of the "standard" lens bags I've seen/used over the years are set up for a 58mm diameter lens, so the f/4's 67mm will probably be too big, and you'll then have to move up to the "pro" lens bags whose diameters support a 77mm lens.
BTW ... see my "PS".
However, when shooting, it functionally is that extra +10mm of diameter in the glass that affords the faster glass. YMMV if you need it, but in my personal view (note: I just picked up the f/2.8 IS), it is when you get into lowlight conditions ... sunrise/sunset golden hour ... that you get frustrated at a system that's no faster than f/5.6 after you've put on a 1.4x teleextender and/or polarizer.
...but I would prefer that option if the choice was given to me.
Everything is always going to be a trade-off, but I've found that having limited options is generally what drives one back to B&H to go buy something better...but its not really that its "better", but is a different trade-off. For example, the f/2.8 IS is a really fast system for low light conditions (especially after a high ISO is dialed in with a dSLR), but the trade-off is that it is an expensive and heavy lens. Sometimes, you'll want to stick with the 75-300mm f/4.5-5.6 IS because its much lighter (and less expensive too).
-hh
PS: I've been having some challenges getting a camera bag to fit my new system, and as a result, I now have a brand new Lowepro "Off Trail 2 Camera Beltpack" that won't fit lenses larger than 58mm in its outer pockets. If anyone's interested in it, let me know. Otherwise, its going to have to go back to B&H as a used item...I'd keep it for my old 35mm system, as my 75-300 will fit, but my wide angle lens is 77mm, which doesn't. Argh.