Okay well, I don't have any personal experience of A-Levels, as I do the IB, but from what you said, it's not really easy to give you any definite advice. For a start, in the IB you'd be taking more subjects, so how do you think you'd be at dealing with the extra work? Also, the IB means that the subjects you take will be spread out amongst different disciplines - you'll have to take two languages, a science, maths and a humanity (history, geography, economics, business) as well as an extra choice - if you want to specialise and focus on (for example) maths and science it might not be the best system for you.
I think from experience, and from what I've read, if you don't have a definite subject path in mind and want to keep your horizons broad for now, then IB will probably serve you well. A-levels, (again from what I've heard) will be better for you if you want to focus on subjects.