Hello RW,

I think that Xp-30 is much more powerful synth then Xp-50 sound wise.
It has a regular set of sounds as other XP models and in addition Roland included 3 of SR-JV80 expansion cards (techno, session and orchestral). Each card costs around $200.

Also, as you noticed the keys feel better.
Basically the xp-30 has the best sounds from all XP series.
If you don't need a sequencer and you like Roland sounds that is the right choice.

Cheers
D.