I think it's best to take Math II, even if they don't require it. At least that is what the counselors at my school say.
Well, you should take the test for the subject you know the best. If you just took a physics class and you did well in your class, sign up to take the SAT II physics test as soon as possible since you will still remember what you learned. Since you did not do well in chemistry, don't take it. Just take what you feel comfortable in, regardless if chemistry is easier to prep or not.