Zynthian looks very interesting, especially as I already have a PI2 and HIBerry DAC+ 
it appears to be based on a PI2 (or 3), and appears to glue a few existing open source pieces together, which is cool, as this is tricky in itself. looks like a a nice PI solution.
its seem to be aimed as a more ‘out of the box’ instrument, i.e. providing some ‘standard’ linux synths/samplers rather than patching synths.
(axoloti is a virtual modular synth environment, so you make your own synths, midi router, sequencer or whatever)
implementation wise…
its a different approach to axoloti which runs a real time os (chibios) which is very light, and has very low latency, which you cannot achieve with linux systems (*) . the other issue with PI systems are they are relatively slow to start up , and the SD cards do fail.
on the flip side a PI has a lot more cpu power, and is obviously a general purpose OS…
(there is a more detailed comparison on the axoloti site)
theres another solution that is somewhere between the axoloti and PI :
Bela (http://bela.io) , I’ve got a couple of these on there way to me too… Bela is a a cape for a Beagle Bone Black, but this runs a hard real time linux kernel called Xenomai, and the cape provides audio processing in that real time context.
Bela, I think, is more aimed at ‘instrument makers’ but does interface with PD via Heavy or C++.
Bela’s site also has a good comparison of alternatives, including contrasting axoloti.
Its great to see a few different alternatives coming along, as these devices are great little portable musical devices… Im getting quite a collection now 