I have 6-core nMP right now. I'm about to buy 2 retina monitors for it, but also considering the iMac 4.0ghz. The Mac Pro does very well with multi-core benchmarks, but since 90% of what I do is heavy VM processing, would the multi-core be better than single-core? I'm looking at getting the best performance possible. So the iMac would be topped out. The nMP is topped out too (well, it's a good middle ground between single/multi-core performance.. 6core). Memory and IO is not a problem. Both are topped out.
