Yes, I would say it is definitely better to compare the XP-30 to the RS-5, a better comparison for the RS-9 would be the XV-88...... Now about the sounds, I can't say much except that the RS's sounds come from the XV series which has some of the greatest sounds I've heard.... so, for $699.99 you can't go that wrong with the RS-5 (or if you want a cheap 88-key, with the RS-9) Also, I don't know about the XP-30, but the RS series have the XV's arpeggiator which is not fully customizable, but is pretty good nonetheless.
Don't forget about the many features besides sounds, if someone does get the RS-5 or 9 to replace the XP-30, they may lose a feature that they really liked on the XP..... even though they're getting some good new sounds. Take a look at ALL the specs before you buy anything, see if there's anything missing that you REALLY need to have.

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Romans 6:23 For the wages of sin is death, but the gift of God is eternal life through Jesus Christ our Lord.