Basically, you've got to use YOUR ears, and YOUR knowledge when making this decision. It isn't about the money (unless it is SO tight that this post is moot, anyway), it's about what makes YOU happy.
Every arranger has strong points and weak points. No exceptions. Roland have weak points (go to Roland-arranger.com for MY opinions!), and Yamaha do too (just don't ask Ian
), but if you've played them both and Roland float your boat, then have at it..!
The online demos of the G70 really show how much guff is spouted at this keyboard. In the right hands, it can sound as good as any other TOTL arranger out there. Better, in a 'live' sense than most, only the guitars are a little weak in comparison to Yamaha, the drums blow just about anything else away, and the piano is without equal (IMO).
If you like to tweak your styles or SMFs, once again, the Roland is by FAR the easiest to do this. If you want, OTOH, to pull it out of the box and do little more than play the styles, Yamaha may be the better decision. I think Yamaha have the best 'instant gratification' factor, but Roland and Korg offer more in the way of customization. Yamaha also have a VERY 'polite', polished sound, whereas the Roland's and Korg's sound more 'live'. If you intend to play primarily as background music, Yamaha may be the best choice. If you want to be noticed, and have a more realistic 'live band' sound, go Roland.
Finally, one last word. The Yamaha's are GREAT OMB arrangers. But that's it... The G70 is a GREAT real live band keyboard, with a full 76 action (the best in the business), an awesome piano sound (ditto, IMO), the best Hammond sim in an arranger, and a killer OS for live use. If you intend to do MORE than just play OMB, perhaps actually play with real live people (I know, I know, the horror!
), then the Roland is by FAR the keyboard that will take you further.
The G70 will give you room to grow. The S900 is what it is... an arranger, pure and simple. Lousy keyboard action, no balls to keep up with a live band, everything optimized for the OMB...
What do you want to BE...? (not what are you NOW)...