If I remember correctly, SATII's were far easier than AP exams, but there are lots of classes SATII's won't get you college credit for because of that. To be quite honest, I'd take every single test that might place you out of a class. That means all the SATII's you think you might have a shot at doing decently on, all the AP's available to you, and maybe some placement tests once you get to college, if they offer them. That'll make your college workload harder overall by eliminating a lot of really easy classes, but it'll also let you get to the things that interest you more quickly. If you're looking for a relatively easy road to a high GPA, the test-out-of-everything approach probably isn't for you.
In case that wasn't clear, I say you should take lots of both. Even if you're doubling up (AP Bio and Bio SATII, for example), you're upping your chances of testing out of those classes.