The reason I'm not too worried about receivers who 'bail out' their QB is because I see Flacco as a Peyton Manning-type of QB who can deliver the ball to the exact spot his receiver wants the ball, as opposed to QB's who throw to a window where the WR may need to adjust his route. Even in games where receivers do not make the catch, I've always felt that Flacco places the ball perfectly to either be caught or fall incomplete.
Either guy would be a great pickup, and you can throw Maclin in there as well. The one reason I favor Harvin is because he's an overall playmaker in the mold of Reggie Bush, and would be able to help in all facets of the game (special teams, short passes, reverses, running the ball, executing the option, etc.) Granted I'm a huge Gators fan, so consider me biased (but also knowledgeable on Harvin's style.)
For what it's worth, Harvin was listed as the #1 overall player when he came out of high school (source: Rivals.com)