Right.....before anyone tries to have ago at me (which seems what this thread has turned into!!) I and NOT currently a med student. I have just done my AS levels in Chem, Biology, Maths and Physics.
I think i will most likely drop physics for A2 but much of the stuff covered in the physics AS course supported work in the chemistry and maths courses. Obviously, this will vary depending on which specifications you do.
There are probably similar advantages to RS. In my biology course, there is quite a bit on ethics (stem cells etc) and if you're hoping to do psychology maybe it will help with that?
At the end of the day you have to play to your own strengths. RS is considered the "easier" of the two and maybe it does show more breadth in terms of essay writing skills etc but physics helps with other skills (as mentioned above)
The usefulness of physics on a med course will depend on where you apply and how well you understand it, and i expect (but do not know that there will be elements of an RS course which may be useful) All med courses have to cover exactly the same stuff in the 4/5/6 yrs.
I chose physics because i enjoyed it at GCSE (but not AS level) and I knew that i was good at it.
Sorry this was so long.....i hope it helped