What I've been told by people at my current firm is that it's not easy to make the move within the MC firm and I should try to get into one of the larger US firms with secondments. If I don't manage to get a US TC, I'd probably just go for the MC and try again later rather than wait another year.
What I'm thinking is that in transactional seats they aren't hugely concerned about where you are qualified. A British-trained US-qualified lawyer might even be useful in some cases (though it's probably extremely competitive). On the flip side, there aren't many fast-growing UK firms in the US, and even so they may prefer hiring from established US firms.
More money is of course great as I absolutely think MC lawyers work almost the same hours as their US counterparts, but my endgame is nonetheless moving to the US.
Yes, I'm a law student. From what I've heard the larger transactional focused US firms have at least had some people qualify directly into US offices in the past. I'm under no illusion there will be any clear path whichever way I go, but I also think going on a US secondment could open some doors.