I shot quite a bit with the 35mm f/2 & found it wanting just like some of the previous posters. Colors were blah (we can fix that, right?), corners weren't very sharp, blah, blah, blah.
I thought I'd get the 28mm 1.8 to upgrade. And it was better. But after a week or two it was buried in my bag & seldom made an appearance. I eventually sold it.
BUT, I just picked up the 28mm f2.8 is. I've taken maybe 400-500 photos so far, in various situations, and although I wouldn't say I've throughly tested it- I love it! I heard a lot of "too slow for a prime", "don't need the IS", "why did Canon even make this lens?" And I agreed with those thoughts (at least the 1st two). But I found shooting with it much different than what I thought. Sharp wide open, very good colors (a lot less post work) & the IS does come in handy given the relatively slow speed for a prime. Honestly, I can't think of a situation where I'd rather go back to 28 1.8. And of course the fact that I paid about the same or less for the 2.8 ($400 brand new) than the 1.8 is currently going for on a number of sites certainly helped.
Since you seem to be sold on the focal length my advice, go to you local shop and test both lens. Take some test pattern shots with both lens. Shoot some inside, lower light photos. Be sure to shoot some test shots with bright colors & some outdoors shots (if you and your camera guy are on good terms).
After you get home and compare the shots I think you'll know which one you'd prefer. Or just buy both when you find a good enough deal. Shoot for a few weeks with each and sell the one you don't want. I found that almost always makes the decision easier, and often cheaper, than renting..
Good luck & happy shooting with whichever lens you end up with, take care.