If they are running a promotion and if you qualify, an auto loan from the dealer's financing arm is always going to be a better option. Dealer finance companies will have subverted rates which are below market as an incentive to purchase their car. As long as you qualify, this is the best bet.
Most credit unions are in the 1.9 to 2.9% range though on new and used cars, so don't discount them entirely. If you can get a better rate through your financial institution, do so and try to find some cash back offers from dealers. Sometime you have to choose cash back or financing terms... if you already have your financing elsewhere, makes the decision easy. Good luck.
Starting Score: 627 EQ, 621 TU - 11/15/08