Zuki,
keep in mind that all those G-70 style files have been converted for use in your PSR-3000, and in some instances, the styles sound as good, or better, than the orriginals. One of the reasons I have not updated my 3000s is the availability of third-party style files for Yamaha keyboards. There are thousands upon thousands, many of which are beyond your wildest dreams. Each time I get a new batch of styles it's almost like getting a new keyboard.
As for the Roland OS, it is no more complex than the Yamaha OS for your 3000 or the Tyros series of keyboards. However, it is very different when it comes to navigation on the fly. Therefore you would have to anticipate a significant learning curve before you would want to take the board out and perform for your audiences. I only played around with the G70 for about an hour, so I am by no means an expert.
The only other objection I would have is the weight. I'm gettin' too old to lug anything heavier than the PSR-3000 around.
As for upgrading to the S-900, after playing the S-900 I found there were some really neat features, but there were some drawbacks that would be a problem when compared with the PSR-3000. This was especially true with the way the LCD display was angled. I guess if you stood up and performed the view was OK, but if you sit down, it was nearly impossible to see without tilting the keyboard up in the back. Additionally, I would have thought Yamaha would have installed the USB port in the front of the S-900 where it was readily accessible--they missed the boat on that as well.
Good luck on whatever new keyboard you decide upon,
Gary
------------------
Travlin' Easy