Requirements for A Level Maths

First time poster,
A student of mine is wanting to do medicine and has just sat his AS in Maths.

He is trying to work out if doing M2, which is an A2 module, would be received more positively by the Medical schools than S1, which is an AS module. He has the opportunity to do either and both are valid options for the A Level.

For information the exam board is Edexcel, if it makes any difference to the advice.
I would really appreciate the guidance of the community especially if there is anyone from the Medical School admissions teams that can advise.

Thanks

MD