my check engine light was coming on and when it did the engine started misfiring and my exhaust would really stink so i took her to a mechanic and they told me my MAF sensor was broke and the car needed a tuneup. they replaced the sensor and gave me a tuneup, the car did run alot better but the light was still coming on and she even shutoff on me so i took her back and they said they would reprogram the sensor and that i should use 94 octane gas for a couple weeks and it should be fine, a friend told me to take her to the pontiac dealership but i know they'll charge an arm and a leg, so my question is should i take her to the dealership.

You're right. A dealership will probably charge you an arm and a leg. I wouldn't if I were you, unless your mechanic can't figure out what's wrong with it and you need it fixed in the worst way.
My friend's g/f has a newer cougar and took it to the dealership and they told her she needed two new cats...and they would do it for something like $1200, no exaggeration! I took my Z to a muffler shop and put up a new high flow cat for $225. He called me to make sure the dealship was full of shit.