I should mention that the "extreme" laptop cpus aren't really that much better. They cost twice as much (meaning Apple would charge quite a lot to maintain their margins), and they have a worse battery life for a moderate cpu gain.
They come out relatively close on some cpu intensive tests, but it varies. Barefeats did some tests a while ago. This is one. Note the 6 core mac pro did come out okay on well threaded processes (after effects). Audio seem to have a pretty massive range of requirements, but the imac doesn't offer much that you can't do on a macbook pro unless it has superior stability on larger workloads or something of that sort.
My suggestion would be that that it come down to whether the OP intends to own a laptop either way. I'm just not sure an imac + macbook pro is entirely necessary. As for the mac pros, the single socket models are really overpriced, most likely due to low sales volume. I do really like them, but I wouldn't say the current model is the one to buy.
http://barefeats.com/imac11f.html
Just my thoughts...