I was just curious about using just a rear stabilizer bar instead of a F/R setup. I had one guy tell me to do just the rear to give me more oversteer ( which sounds like fun ) and another guy tell that i should just do both. I just want to cure the understeer of my car.

Which setup would be better for canyon driving?

If you want no understeer and would like to the the rear end out more, don't drive a FWD car.
A larger rear sway bar will help reduce understeer, but does have the potential to screw up the handling big time. How big you go will depend on several factors - what's available (even better if you can go custom made like I have) and what will suit your driving style. Ultimate would be a blade type rear bar with in-car adjustment, but not always practical on an everyday car. You may also need to alter the size of the front bar too to keep a good balance.
It's not always a simple 'bolt on' job unfiortunately. My advice would be to read up what you can on the net and see if someone else has done similar mods and get them to take you for a drive so you can some idea of what to do. If not, you have the potential to upset the handling to a point where the car may become almost undriveable
Gary