There is certainly an overlap in content, particularly between the mechanics sections in physics and the mechanics modules in further maths, but on the whole they are quite different subjects. Further maths questions tend to test the physics concepts in a more mathematical way, so no wordy questions, less understanding, more equations/formulae involved), whereas physics questions tend to be rather simple in terms of calculations however the phrasing is harder to comprehend hence there is a lot more understanding required. I would do both but don't think that simply nailing the mechanics modules in further maths guarantees the same results in physics mechanics, not that easy.