I did both .. definitely go for S3. No doubt.
Also since S3 is an A2 module .. it is an extra module which can help towards your A*. With D1 you will probably have 3 A2 modules so you will need 90% across them three to get A*.
If you repace D1 for S3, you will have 4 A2 modules in FM, you have to get 90% any three to get A*. So if M2 turns out to be a bad paper and you mess up, still have an extra A2 module to rely on