The 70-200mkii is a great lens and if you find that at 135mm it's close enough to the 135/2 then by all means do it. I have both 5d models and the old 5d "classic" is adequate for me. I too am considering getting rid of the mkii as I don't need all the resolution and ISO1600 on the classic is good enough.I'd say rent the 70-200mkii and if it you like it on the 5dii then you should like it on the classic provided you aren't always shooting at high ISO. Primes are great but if you need the convenience of a zoom then I'd go for the swap.

