I just got the new LX5 and also was able to spend some time with an S90. The LX5's new scroll wheel on the back partly fixes the #1 usability bug with the LX3, namely the inscrutable decision of which joystick to use for what.

Comparing the LX5 to the S90, one difference immediately came out: the LX5 has a much better macro feature. The LX5 can happily shoot one inch across, while the S90 can't go much smaller than two inches.