Hi Silva,

The RD700SX seems to be a really good piano, I haven't played it though, but I do own a Fantom XR which has got one of two main pianos included on the RD700SX (The Ultimate Grand sample).
I think this sample is one of the best I've heard, I prefer it over the Yamaha's (Motif ES & P-series),I can't really comment on the Superior Grand sample as I haven't played it, but the online demos sure sound good, I might have considered the RD700SX if I haven't had my Fantom XR.
Also, the RD700SX costs a few hundred euros more than the Promega 2(2000 vs. 1695).
The advantage of the RD700SX is that it includes a lot more sounds than most stage pianos, would be really useful if you need more than just piano sounds.

Greetz ,
Marcel