Since you did physics AS, (assuming you took a board with a mechanics paper) you've likely covered 50%+ of the material already. Check it out: if you haven't, or found it difficult then do the other one.
I did M1 and D1 last year, personally I found M1 a lot easier because it has a lot more maths in it, and each process can be thought out logically. As long as you consider all the forces, it's fairly simple. D1 could be compared to "professor Layton puzzles for mathmaticians" if you like - what is the shortest route between A and B, etc (good for programming so I hear).
It depends on what you'll find easier. If the mechanics in physics is what brought your grade down, I'd lean towards D1 instead.