The inputs may be a better quality, but nobody knows, they just take hotter signals, that’s all anybody here knows iirc … there’s no advertised changes to the output side afaicr
you missed adding that the illuminated buttons contribute to the added value though 
if it’s circa $300 difference new, then I suppose it’s not too much more, I think the added headroom is definitely helpful, but no show-stopper, remember, whatever the signal strength and whatever you do with it, as soon as it’s hooked in it gets a 12dB attenuation anyway
I don’t think parts availability wrt the MKi is something to worry about at all fwiw
I think the case for a new MK2 against a good value s/h MK1 is possibly a trickier proposition, either way, the announcement of a MK2 was good news all round for all OT owners imho