I have owned both and sold my 24-70 f/2.8L I for the 24-70 f/4L IS. For me and what I shoot, it was the perfect call.
If you need to shoot portraiture, events, reportage or sports, get the f/2.8L I. (
War reportage in particular -- it's built like a tank!)
If you need to shoot landscapes, macro, video, if you travel or hike with it, if you are space-confined in your bag, etc. get the f/4L IS.
But if you want to get in the weeds, here are the upsides of the 24-70 f/4L IS:
- Sharper
- Lighter
- Shorter length
- Has IS
- Has a much smaller lens hood
- Has a 0.7x macro mode -- a killer feature for travel/hiking work: leave the 100L at home!
And the upsides of the 24-70 f/2.8L I:
- Opens up to f/2.8
- Has a fairly rare reverse telescoping acton, when coupled with its comically large lens hood (honestly looks like a 70-200 f/2.8 hood!) can optimally shade from flare throughout the zoom range. Every other 24-something Canon sells only optimally shades the sun at 24mm.
- Built like a tank. Shockingly solid feeling.
- Probably a slightly better bet on the resale market as an f/2.8 standard zoom is a staple pro tool.
- A