If you could afford the 5DIII then why even look at the 28-135? Go with the kit lens, 24-105. Very good value for the price. The 28-135 is an older lens, slower than the 24-105, and isn't weather seal/dust resistant.
I agree with Juga, I've shot the 28-135 on my 5d, and it's not even close to quality of glass of the 24-105L, which I ended up buying over the 28-135. It's not a bad lens by any stretch, but with the weather sealing and the constant aperture, plus the L glass, The extra 100 bucks for the kit is a no brainer