Ah, high school; well that changes things a bit.
If you have plans to enter college and shoot for a computer career, I'd definitely say do the computer course and try for your A+. As you deduced, A+ is basic, but almost all entry-level tech support positions will require it. Upon entering college, grabbing a job through Best Buy or similar service will do well for experience as you pad your resume with a degree.
If you're not sure if you're going to college, or plan on doing something other than computers, then the A+ isn't as appealing. The other experience can be more beneficial depending on what it is.
And in the end, any certification you complete can always still sit on a resume no matter what career you end up going into. It still adds a good alternative angle for employers to see.