I will be in Y10 after the summer so I will basically have 4 or 5 years (until Y13) to prep for SAT or ACT. Just not sure which one to take. 5 years --> study for both?
Both the SAT and ACT are based on mainly GCSE level material (with some aspects being AS level, and AS level analytical skills obviously would help; also subject tests are closer to AS level as I understand), so for starters you really don't need to spend 5 years preparing for them (this is also a damning indictment of the US education system incidentally...).
Both are acceptable - I'm not aware of any reputable college that doesn't accept both. There are differences in them - the ACT requires slightly more advanced mathematics (matrices and complex numbers notably, and has slightly harder algebra questions - I'm fairly certain there's no calculus in it though) and has a "science" section which focuses on data analysis and logical reasoning (no specific scientific knowledge is required). Or at least, that was the format back when I was looking at them...the SAT on the other hand just has Maths and "Critical Reading", which more or less is based on GCSE Maths and broadly the analytical skills you'll have developed in GCSE English. I mean the CR section when i took it was pretty much like the 11+ type style of reading comprehension questions (also there were vocab questions although this might have been part of the now defunct writing section).
I'm fairly certain Writing is no longer in the main SAT, but I think might still be a subject test? In either case you usually need to take 2+ SAT subject tests - usually one of the Math tests and another (I think Harvard used to ask for three, and e.g. MIT/CalTech want the second one to be a science - you can take up to 3 subject tests in a single exam date, but you can't take them on the same exam date as the main test. Make sure you'll have completed both in time for you to submit an application - this may entail taking one in the summer and the other set in the fall (this doesn't matter so much if you take the ACT and 2-3 subject tests as you can do them all in approximately the same time period, as the dates are usually set up to avoid clashes). If Writing IS a subject test, some colleges require it - I would recommend planning on taking Writing, Math, and another in a subject of strength (don't take US History if you didn't go to a US international school and took a US History class in the style they teach it in america).
They should both have a practice test (which isn't a full test - which takes about 2-3 hours, but a reduced version for you to get a feel for the types of topics it covers and so on) which is scored on their website. I would suggest at the end of year 10 to take these, see which one you prefer and do better on (although keep in mind you'll probably do better on ACT Math after doing some AS level Maths/FM or at least learning those specific topics - also the matrices/complex number topics that come up are pretty simple, I don't think they'd ask you to do anything with more than a 2x2 matrix unless they give you a really easy 3x3 to find the determinant of (i.e. one which has a row/column which is has two 0s in it or something) and the complex numbers is just basic ideas about complex algebra.
All of the above may be out of date and/or irrelevant though but check the websites as the practice tests will give you a better idea, although hold off trying them till after you at least finish year 10 or 11