I've used both with Live and for me there are three main workflow issues which would determine which one might be better for you.
First, how much CPU power do you really need for your projects? If you're a heavy plug-in user and you tend to use multiple instances of CPU-intensive synths, then a Mac Pro will will give you a bit more breathing room before you've got to start worrying about freezing and/or rendering. If you tend towards simpler projects or tend to work mostly with audio a newer iMac would be just fine. Not knowing which or how many plug-ins you use or what sample rate & latency you need to work with it's hard to judge one way or another on this.
Second, do you use a lot of 3rd party plug-ins where you want to see their floating windows open at all times? This was the deciding factor for me, as I wanted to run dual monitors because I like to monitor the levels on multiple tracks' plug-ins when mixing. Plus I also use Logic, which is quite happy spread across two monitors. Yes you can add an external monitor to an iMac but matching monitors are kind of cool. On the flip side, if this isn't something that's important to you a 24" (or bigger) iMac makes a pretty stellar Live machine - especially considering the current either/or interaction between the session and arrangement views.
Third, what kind of audio interface do you have/want? PCIe is still probably the king as far as bandwidth and stability, but I'm running 24 channels on the stock FW port with no issues at all. As long as you don't want or need the PCIe bus for your interface, either will work just fine.
Most of the other differences are also relatively moot. Mac Pro's do allow for massive internal storage but FW drives are almost as cheap. The ability to run multiple graphics cards isn't really an advantage for audio work. Bus speed is a bit better on the Mac Pro, but again it's splitting hairs unless you're really pushing the envelope.
Honestly, if I hadn't been hung up on the dual monitors I'd have keep my 24" iMac. But I admit the seemingly unlimited CPU headroom is kind of nice too...
Good luck with your decision!
