I think if you look around, a lot of people compare the S90 to the "G" series regarding the sensor and the quality of the images it produces. Doug is right, the G series is probably the best you can get in the "P&S" range. For me it was just a little too big in size.
I'm know there are differences like I think the "G" series has more zoom, but for me it wasn't worth the increase in size. I have DSLR's for that. For me, I wanted something small, that shot RAW, and didn't have the noise issues a lot of other P&S cameras have. If you don't have a DSLR, don't mind a slightly bigger camera, and want the most control (thought the S90 has manual as well as exposure compensations so it's pretty capable to override and get what you want out of it) and quality you can have in a P&S the G series is hard too beat.
FWIW - do it right the first time. The upgrade game cost me lots of money in the past. In the end it costs more to do it twice than just shelling out and doing it right the first time.