The iPad garage band is pretty good, but is a bit more of a sketchpad than a full DAW. You can get pretty decent recordings on it, but I think you are limited to 8 tracks, and some of the menus aren't quite as intuitive as the desktop version. Also if you use virtual instruments, as you add more and more of them, it takes up more space, and an iPad has a smaller harddrive than a desktop/laptop (unless you have a macbook air, in which case they are similar size HDs)
I've not used the full desktop version enough to find all the differences yet, though from my dabblings it is more in depth than the iOS version.
As for peripherals, I use an iRig (looking to get the iRig HD soon) as I mainly just record guitar (can't sing worth a damn), but it is a good little bit of kit, easy to set up, and I carry it in my guitar case so where ever I have my guitar I have a whole band in my pocket, and more virtual amps than real ones. I also often use Amplitube (again more guitar specific, but they are expanding bit by bit, though you have to buy the add ons).