Well, Kingfrog, if it was me, I'd first be thinking of warranty, so anything used would be out.
I am planning on using the same setup as you suggested...a PSR-S900 and a P85 digital piano...both weigh around 25 lbs, the latter has MIDI and a lovely Graded Hammer 88 note keyboard.
The G70 and E-80 are fine instruments, I must agree, but with the S900 and P85 you will have far more flexibility (and lots of 3rd party support)PLUS a warranty.
I give them THREE thumbs up!
[This message has been edited by ianmcnll (edited 04-07-2008).]