Well reading your reply it 'sounds' as though you're more interested in physics. I am myself biased towards physics so take what I say with a pinch of salt. But try to imagine yourself doing mechanical engineering. There would I'm sure be a lot of practical work in which you'd have to build prototypes of engines and systems, and as in all walks of engineering, it doesn't matter how good your theory/plan is, if it doesn't work in practice then it's no good.
Physics on the other hand is often about imagining perfect systems which can be solved exactly (eg for quantum mechanics, see particle in a box, quantum harmonic oscillator and particle on a ring). For the enthusiastic student there is a certain amount of natural beauty here as it's very satisfying to be able to find exact solutions for systems. Of course in real life these conditions are hardly ever replicated, but e.g. for my subject chemistry we also learn these perfect models and then later learn to modify the conditions and approximate reasonably answers, with the models still being a good basis, and conceptually useful.
So just have a think about what you really find interesting, Imagine you're at university chatting in the bar. If some of your mates are talking about the merits of big bang vs steady state theory, are you going to feel left out if you're studying mechanical engineering? On the other hand if you're not fussed about intellectual snobbery and want to (literally) get your hand dirty and make something that's going to have a direct impact in people's lives, then ME is an excellent choice. But in my opinion it's always best to choose the thing you're most interested in to study, careers and money can follow on later.
Just to touch on your query about transferring to ME later, that's something you would really need to research e.g. by directly emailing some physics and ME tutors. To my knowledge there is a certain amount of transferrence that goes on between science and engineering, but the more common ones I've heard of are chemistry -> chemical engineering and physics -> electronics engineering. I think you need to be more careful about physics -> ME as the latter is all about designing things that actually work in practice. I have some physics friends who are geniuses, yet couldn't change a lightbulb if their life depended on it. So I think transferring from a highly conceptual subject to a highly practical subject is less common, although I think there may be conversion courses.