If your aim is to do CS then FM is probably more useful than A-level CS, assuming you can get similar grades in both (which is a considerable assumption to be fair!). It's also probably more of a practical workload with your other subjects, as 4 disparate subjects is, I gather, a lot more work than two unrelated subjects and double maths.
In terms of whether it's taught over one year or two years, this is down to your school. It is an A-level and so formally designed as a two year programme. However because it necessarily needs to be taken with A-level Maths and a good chunk of the content depends on content from A-level Maths, a lot of schools plan their maths/FM teaching so students do all of A-level Maths in year 12 and all of FM in year 13, which I guess makes it simpler to schedule when you're teaching each topic (as that way you know the FM students have covered everything they need from A-level Maths).