SL is missing a few features from SX, I think TrueTape is gone, the MPEX timestretching is gone, you can only load 16 VSTis at once as opposed to 32 or 64 or whatever it is in SX, etc...
Basically it's plenty functional but some of the bells and whistles are missing. You can find a complete list of difference at www.steinberg.net .
You probably won't find as good a deal as the Mars price, since they've been going out of business and liquidating their inventory...