I bought the XV-88 but they're both great. I like the keyboard action on the XV better (more piano-like to me) but the "stock" piano on the S80 sounds slightly better IMO. However, the SRX-02 Piano expansion board for the Roland sounds fantastic. I will buy one as soon as I can afford it. Haven't had a chance to hear the Yamaha expansion cards. I'd give the edge to Roland on the EP's, plus there are more of 'em.

BTW, you might want to check out the 'XV-88 vs. S80' discussion at The-Gas-Station.