hi Dan,

given a choice between those two models, the 5D Mark II is going to give you much better performance. It will be better in low light, and you should get sharper pictures.

 that being said, the lens you buy is just as important as the camera body.   something like a Tamron F2 .8 24 to 70  will give you much better pictures than a cheap kit lens. I mention the Tamron in particular because it is less expensive than the comparable Canon, and I think it's a better lens. It's the one Bill and I both use  for our walk around lens. 

 The smaller sensor will have no effect on lens compatibility.   as far as the  difference between a metal or a poly body,  neither  has any effect on picture quality. 

hope this helps. Let me know if you have anymore questions, and I will try to help if I can.

Tara 

"If your pictures aren't good enough, you're not close enough."  ...  Robert Capa