Definitely try the sp404 and
download the manual before committing.
It looks real good on paper but has a lot of limitations -- for example, it doesn't set tempo in 1 bpm increments throughout its whole range. To my ear it doesn't sound all that great either, and once you take a loop out of its original tempo the audio can get downright nasty -- this is not Ableton Live.
There is no MIDI out jack, btw -- so you would have to sync it to another device, no way to send clock from it.
But see what you think. People unload used 404s regularly. I ended up buying a used 606 for about the same money, smaller maximum memory but it's much more professional in its feature set although the audio issues are the same.