Depends on how big she is and her skill level. If she's short, can she reach the pedals of the tricycle? Can she pedal or learn to pedal easily?
Both are winners. DS got the cozy coupe from Santa last year when he was 15 months. He was so funny. He jumped through the window Dukes of Hazard style LOL. He got a tricycle in September for his 2nd birthday. I was surprised he learned so quickly how to pedal and steer. He still loves both.
If you have a Toys R Us nearby, take her in and let her get on them both and see which you think works best for her.
We have both. I like the bike better. However, it is more work for us b/c he could not peddle at first and we had to push him (it has a handle for pushing). But my LO learned to peddle early, he's been riding a bike with training wheels since he's 2 1/2