Hi FAEbGBD:
As far as I know, the RD-700 is more like a digital piano, while the XV-88 is a synth with 88 keys. The strange thing is that the RD-700 can use the new SRX-boards, and this means that it must have basically the same synthesis as the XV-series.
But, if I'm not totally wrong, you can not edit the patches of the RD-700 to the same extent as with the XPs and XVs.
So, if you need the "synth features", I would go for the XV, but if you just need a good keyboard with great sounds, then the RD-700 would probably do the job.

The RD-700 is probably one of the strangest instruments that Roland has ever made, being a hybrid between synth and digital piano..
Stig