Which quad core Mac Pro? The current one?
Compared to the current Mac Pro, I would say that the new Retina Macbook Pro probably has a slower CPU. It's hard to compare since they're different generations and different processor lines, but the Xeons are very high end.
The Retina MBP has higher clocked RAM, but less total RAM capacity.
I think that the GPU in the Retina MBP will perform better than the base GPU in the Mac Pro, since that's from 2009.
The Mac Pro wins hand down if you want easy expand-ability. Lots of room for hard drives, SSDs, and expansion cards.
The Retina Macbook Pro does however have two Thunderbolt connectors. Thunderbolt is basically external PCI-Express (plus Displayport). The Thunderbolt chip used in the Retina Macbook Pro supports up to 80GB/s of aggregate data bandwidth (40GB/s in each direction) over Thunderbolt.
