You say you have to work on heavy 3D projects, if this mean that you work with SU professionally than I wouldn't recommend both cards.
SU is using OpenGL and the better option then would be ATI Fire 8700 or NVIDIA Quadro 5500. These graphic card have full OpenGL support and better drivers. They do cost a bit extra but it's worth the extra money. It doesn't matter which one you choose, they are both very good for 3D modeling and rendering.
You can check here for a review:
http://www.tomshardware.com/charts/workstation-graphics-charts-2008/benchmarks,3.html
Check here for SU and OpenGL:
http://download.sketchup.com/OnlineDoc/gsu_mac/M-Technical_Reference/Tech-OpenGL.htm