I love using nanoloop on the micro, the screen is great, if a bit small, and I find it surprisingly comfortable to use. My only complain is that using it for recording there's a terrible terrible buzz that comes through, just loud enough to be annoying to deal with.
However, for composition, I love using it because it's so pocketable, there's the built in headphone jack, and it's just nice and convenient.
I have, however, been looking into getting an SP and doing Nex's "prosound" mod. Also because the SP has a much more standard link cable adaptor (the color and GBA cable work with it), whereas the micro has a different one. Having to deal with that makes syncing with LSDJ almost impossible.
Realistically I got the micro I did (the famicom anniversary edition) because I liked how it looked. It just turns out to be good for this though. Hope this wasn't too rambly