We are in the same boat on this one, as I just went through the 24-70 shuffle. Had the f/4 IS version for a few weeks, wasn't too thrilled about some of the performance aspects, and subsequently traded up for the f/2.8 II in the pursuit of image quality (and a "buy once, cry once" mentality I have with all things). Here are my thoughts:
f/4 IS
PROS:
- much lighter weight
- much smaller size
- IS is a great asset
- macro mode is really handy for walk-around/do-everything lens
- price: $1k less than the f/2.8 II
CONS:
- focus shift at 70mm at/near MFD. This really is what prompted me to move past this lens, as I would be shooting primarily at this focal length
- image quality is super good, but isn't quite f/2.8 II levels (more so a pro for the f/2.8, but it has to be mentioned). Except, 50mm is pretty soft. The two copies I tested were rather bleak.
- AF "hiccups". In certain situations, the AF just wouldn't respond, as if the lens didn't wake up with the camera. Both copies exhibited this behavior.
f/2.8 II
PROS:
- The IQ of the f/2.8 II is mind blowing good. SharpSharpSharp
- AF performance is top shelf
CONS:
- Price... ouch. But if you're in it to win it, this isn't a concern.
- Weight
- Size. It's not toooooo bad, but compared to the f/4, it's a behemoth.
Seriously, the f/4 IS is pretty darn good, and if your primary reason for owning a lens in this focal range is general purpose walk around use, no question this would be the lens I'd buy it. The macro mode is a bonus here, even though it isn't true 1:1. And the IS just makes it a winner.
If you're a shooting pro that has to have f/2.8 or win test chart sharpness contests, the II is the answer. The 2.8 II turns in such awesome image quality that it makes for a tough decision, but the weight and girth is a major consideration, along with the gaping hole in your bank account. If I was doing more event work or paid gigs, no doubt this lens would have been my go-to.
I had some bad buyers remorse because I firmly believe that my photography enjoyment will be better enhanced with a 100-400 II
So, I recently returned the f/2.8 II. I'll more than likely add the f/4 IS to my line up at some point, as it just makes for the perfect walk around solution despite it's small quibbles.