My workflow is based on cores & ram (3d art) - I'd stick with the 1,1 because I saw a real jump in performance once I was over 16Gb of ram. And the fact that I could replace the video card with something better made the 1,1 better. As was the ability to add multiple multi-terabyte drives. And and eSata port to add a 4 bay external HD for backup.
If you are CPU and single thread bound, and you can fit everything on the internal HD and such, then the iMac would probably be better.