Quote Bob Bickerton:
A pro flash player may have better converters (not a huge issue for gigging live situations IMO), but in terms of interface usability you can't beat the iPad.
Not quite true for the record, some of those, namely the Tascam players, but a few others, all have direct track access remote 'keypads' that can be placed much closer than the unit itself, and would make it relatively easy to launch tracks as well while having the unit in the rack. Of course this adds to the cost. And of course you could always do similar with a computer which has already been mentioned, even to the point of launching tracks from a keyboard on stage if you really wanted by tying it to MIDI.
All that being said, the point is there are a half dozen ways to accomplish this on stage and several 'good' ways, or at least 'not bad' ways, including sticking with MD as well.