I very recently did that exact switch. I needed the portability for location recording.
Re the fan noise - you can download a free menu bar app called gfxCardStatus and force the computer to stay on integrated graphics when you need the computer to stay quiet. This compounded with an SSD stops most of the fan noise. Then again, I only record with two inputs at a time.
I actually prefer the more intimate screen - sounds kind of weird, but I find it's way more manageable when I'm dealing with sheet music editing. The 15" hits that sweet spot for me.
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Guess it's also worth noting you can get 16GB (8x2) ram for under $100 these days if you keep an eye out. I just did on newegg. And, it's waaaaaaay easier to stick an ssd in down the line on a notebook.
I'd say unless you really don't want to lose the imac's screen (and you don't have the money to get a separate screen), and for some reason you need more than 16gb of ram, the macbook pro is a better investment. Easier to upgrade (you can put two 2.5" drives in), portable (I didn't like being chained to the desk), and not to mention I really appreciate the backlit keyboard.
Anyway I'll shutup now, but this is coming from a guy who did it and loves it. Just make sure you swap out the hdd for an ssd, or at least a 750gb momentus xt. The 5400rpm drive it ships with really holds it back.