3DS XL is the best, 2DS is much more comfortable to hold (better button placement, bigger L + R triggers) than the standard 3DS.
For £79 it's a bargain and worth every penny, there's loads of great 3DS games.
As for the mono speaker thing, just plug in some earbuds and you've got stereo again. Not that the mono speaker is a big deal, still sounds great to be honest.