You don't actually need to choose between the new and old SAT. The new SAT is the only test offered now, so you'll be taking that one in October no matter what.
As for your question on the math subject tests, I'd recommend Math II over Math I. As you might expect, Math I covers less material, and most of it applies to the math skills you would have learned in basic algebra and geometry classes. Math II covers all the material on Math I, but it adds in some more advanced topics as well. For example, Math II tests your knowledge of vectors, complex trigonometry, and different types of functions. Before taking Math II, it's best to have taken classes in pre-calculus and trigonometry. For more on the specific topics that show up on each test, check out this article. Since you've already finished high school, you've probably covered all the material you need to know for Math II in your classes.
Be aware that some colleges specifically require certain math subject tests, so you should check the specifications at schools that interest you before making a final decision. Some schools ask for either Math I or Math II, and some just want Math II. This is another reason why Math II is a better choice - it's more useful overall in the college application process.