Side airbags, as Vi has pointed out, are an incredible safety feature and they can help immensely in a T-Bone type accident. Use your own cost/benefit to decide if they are worth the option price if they aren't standard, but for most people they are.
Stability control is like traction control or ABS brakes, it's a safety net. If you get into a situation where the system is invoked, you've already f-ed up and should consider yourself damn lucky. Unlike situations with airbags, where you can be doing everything right and still have the system come into play, with stability control you are the only factor in whether it is useful or not.
Oddly enough, in a typical governmental ass-backwards move, side airbags are not required and are not scheduled to be required, while stability control will be legally mandated by 2012.
ZV