I think the DSi XL is the best of the lot, there are flash cards available for them. Nitrotracker is probably what I would go with if you're going homebrew, but I'm too tied to my hardware sequencer (read:old+lazy) to do tracking these days.
DS-10+ is the enhanced version of the original DS-10, refer to the other post. It's really a great little package.
Audio quality is surprisingly good, people are pretty spoiled these days. If you want to get crazy I've heard there are guys that modify their hardware to tap the audio before headphone opamp, but I think that's kind of silly. I can guarantee you that the outs on the MS-10 were a hell of a lot noisier then, and certainly are after thirty years. ;)
A DSi XL, a Monotribe (or two!) and a Roland Mobile Cube would make for one hell of a portable synth rig. The monotribes do pulse sync, so you can run DS-10 as master clock. Run the audio channel through the 'tribe's filter and hold on to your butts.