Some schools require them even if you take the ACT (I'm pretty sure Princeton does this). Ideally, you should take Math 2, a science, and a humanities (you are allowed to take up to three tests in one sitting). If you only want to take 2, I would suggest Math 2 and something that relates to what you want to study. You might want to consider taking a Subject Test for an AP exam you have taken. Barron's is great for Math 2. I didn't start studying until about 2 weeks before my Subject Tests, but I was also in the middle of AP studying which helped for my Physics and U.S. History Subject Tests. If you decide to take them, you should start studying now because you are going to want at least a 750 on each test you take.