AS Level Satistics or MechanicsWatch
Statistics has less content (or at least does for MEI), but I personally hate it with a burning passion. Stats also complemented part of my A2 Biology (OCR) course.
I'd personally go for Mechanics, because it's more interesting, but it depends on whether you prefer physics or biology.
I'd recommend you flick through the course outlines online and have a quick look at the content to see if you prefer one over the other.
Mechanics is generally very 'physics-y' so to speak. Stats is more graph-y and data collection (surprise, surprise). I guess it's where your strengths lies and also what you're thinking uni-wise… If you are taking physics at uni, then take mechanics for sure as they both strengthen each other
Do statistics if you want a change from physics.
Lots of people don't like S1, but it's worth giving it a go learning it. S2 is much better.
Edit: I didn't find very much cross-over between statistics and A2 biology. Doing S1 and S2 will only help you on 1% of A2 biology, so isn't really a reason to choose statistics over mechanics.