I've never used Sonar, so I can't compare the 2, but I can say that Cubase SX has always been pretty stable. It also seemed to have everything I needed to get everything done.
One thing I always wished SX had, though, was the ability to further process effects sends- route them to other effects and whatnot. I understand V.2 incorporates that ability, but haven't seen fit to upgrade yet.
I'd go with SX 2, for certain though, if you decide on SX. Full sorround capabilities (I've had A LOT of fun playing with surround in SX1!) and a whole slew of cool features.
For whatever its worth.
Chris