I currently do Maths, Biology, Chemistry, and Physics. They are nothing like GCSE science and maths, but in the best way possible. The content is a lot more interesting (mostly), obviously the workload is a lot greater, and sure there are days when I wish I'd picked different subjects.... But if you love science then by all means go for it. It's 100% possible to work a lot, and still have a social life, you just have to do it in the right way and start revision early (but I'm sure you'll be able to figure this out for yourself next year).
In terms of difficulty? Chemistry has a lot to remember/understand and exam questions are usually applications of knowledge. That being said Chemistry is amazing (though daunting at times), and is probably my favourite subject that I take.
Biology is all about memorising content (and there's so much to remember unfortunately)... Exams require awfully specific answers, where if you don't use the right keywords then you don't get the marks, even if your answer is right... (especially true for OCR)
Physics is not too bad at all (surprisingly), the first module overlaps nicely with M1 in maths (if that's what you'll be doing), and is a combination of a lot of the GCSE stuff and some new material. The second module is a bit harder though, with less maths, and more wordy questions. A lot of application of knowledge, and plenty keywords... But it certainly is fun (especially the practicals).
Maths is kind of a big step up from GCSE, C1 is pretty much all GCSE material (+ integration, differentiation), C2 starts off easy then gets a bit more difficult... M1 (if that's the module you'll be doing) is the hardest, and a lot of people at my school are retaking maths because they failed M1... But if you do Physics, it shouldn't be that hard (it becomes easier with practice).
With any of these subjects it's all about practice. You'll find that more work is required than at GCSE (heck you can get away with absolutely no work at GCSE), but if you practice frequently and revise what you get wrong, then you'll be fine, and left with plenty (less than now though) time to socialise.