The premise of the question already assumes that Prometheus is not a good film. There are plenty of threads already debating the credibility of Prometheus...this isn't one of them. This poll is strictly for those people who already think Prometheus is a bad film.
Both films attempted to be serious with their subject matter but I think that the storyline for AvP is just more in-line with Alien franchise than Prometheus is. The dialogue is better, the character development is better, and the film has aliens in it.
My vote is that Prometheus would not be better than AvP.