Code P0171 for a 2002 subaru wrx, MAF sensor or O2?

I have a 2002 Subaru WRX that the code P0171 keeps coming up. It wasn't bad until the other day when i was on the highway accelerating and a big puff of black smoke came out and i lost most of my acceleration after 3500 RPM. After 3500 RPM the car starts to buck alot, I've been told it's either the MAF sensor or the O2 and this is causing me alot of stress figuring out which one it is. Any help?