Imac is 2009 i7 27", 2tb sys, 2x usb 1x fw800 external discs
8gb Ram, 10.6, cs5
MacPro is early 2008, dual 3.0 quads, 1gb radeon upgrade (stock card failed twice) blackmagic hd extreme, bay 1 system hhd, bay 2, time machine hdd for bay 1, bays 3&4 striped barracuda 2tbs giving 4tb at 270mb/s using software raid- and this is a sataII 3gb bus. Fw800 storage.
With a macpro tower you could have a 256 ssd system drive and striped bays 2,3&4, probably hitting 350-400mbs, 3x barracuda hdds giving massive capacity and massive speed for cost of one 512 ssd.
Top end i7 not costing much less than macpro, but macpro due decent upgrade.