Just my two cents:
I upgraded to the 5D Mark II, from a 400D. All I can say is wow! I have the Sigma 50mm F1.4 and a Sigma 24-60 F2.8, and they are great with the 5D!
The deal-breaker for me was FF/ISO performance, and everything else that comes with that.
For me it was a lot of money (since I'm still a student!), but it was my first body updrade since I bought the 400D in 2006.
Sure, the 5D Mark III is better - but do I need it? No.
It's up to you in the end, but for me it was a VERY VIABLE option
And I'm loving it!
Some people complain about the AF, and well. In REALLY dark situations, like photos in clubs or similar, the AF cannot cope. However, the ISO performance is great in these situation... Solution: buy a flash to use the AF assist grid! I don't use the flash, but perfect focus
At least that is what I did... And for all other situations, the AF was fine for me. And If you want to do evening/night landscape photography, well... there is always the live-view and manual focus...
I only use center-point AF though...
Just my thoughts...
I hope you will be happy with your final choice!