Current Aero-Mech dual here. If I could do it all over, I'd drop the mech part and just go Aero. Why? So that I could take more Aero (and by association math) undergrad classes. I'm not a big fan of the mechanical aspect, don't know if I want to do grad school anymore, and would rather take classes like Comp Flow, Turbulent Flow, FEA, Fluid Mech, CFD etc. etc. than more classes designing bearings/bolts/screws.
I have pinpointed that this is due to preferring "ideas" and gaining theoretical understanding rather than algebraic design parameters. (ie. I'd rather learn about the material science aspect of designing a bolt than just applying a "trust me, this is how you do it," approach.