iMac will be fine in terms of CPU speed. The Mac Pro is the better choice if you need more Firewire Ports or PCI-E slots. Logic 9 64bit just released and it removes the memory limit, so there is no 4GB limit anymore.
Just in case you didn't get the message the new iMacs will give you plenty of power for Logic. Get as much ram as possible and an external firewire hard drive too.
I would stay away from the 2009 Mac Pro. Over in the Mac Pro forum there is a huge thread about the performance and heat problems this Mac Pro has with audio applications...
Also, I have seen some benchmark tests over at gearslutz showing the 2008 Mac Pro outperforms the 2009 Mac Pro with Logic Studio. It sucks because the PCI express slots are an important feature if you want to add on cards for UAD-2 or Apogee.