I have the same Mac Pro that you have, and also have a 2 year old iMac i7. There is no contest. The iMac smokes it on everything except internal storage.
If I were you, I would get an iMac with a SSD, and attach disks USB3 to it for storage. Get 8 gigs of Ram and you are set! I also have a Drobo, and that might be a good fit for you now that they have a Thunderbolt version. Get the iMac when it comes out!!!