I'd say they be around the same speed if you get decent ram in the iMac. Even though I hate to sugest this as an option, if you have a windows version of your software you can use bootcamp, Windows really does fly on them, that's what I'm doing for now. I have OS X CS2 for quick fixes, but longer projects I load up Windows, I havent't tried it though Pararells, but that's different and I don't really care for it at the moment.
Good Luck!