I'm getting a dual degree in physics and philosophy. Can't speak for mechanical engineering, but the combination I have is pretty interesting. Very different in terms of the work you will be doing (ie philosophy is all reading and writing, whereas physics is working problems and whatnot). Both are very analytical (well, depending on the philosophy program you go into - ours is), but it's not like there's much overlap in subject matter - though they're both asking about the 'big questions' so to speak, they go about that in very different ways. Personally, I rather enjoy the contrast, but you have to enjoy reading and writing to enjoy philosophy. Mechanical engineering, huh? Can't speak for that as a major, but physics, in my mind, is at the top of the heap. So, if you can handle it, I say you can't do any better than physics. So, of course I recommend physics over an 'engineering' degree. :p