I was against Lampard initially and am still concerned about the defensive setup he had at Chelsea.
@Eggs nailed it earlier when he said that he could be a better long term option, while Rooney probably gives more of an immediate bounce.
With Lampard he comes across as a 'thinker' and will be aware of the mistakes hes made and has the contacts and background to logically work through them.
I could imagine him building something and growing and improving over time into a top manager. He comes across as pure class as well.
With Rooney, he may be brilliant for the season...but he's going to be football minded rather than someone perhaps who Id say would be a squad builder...so he would likely need a strong DOF to support him.