I entered Andover as a 10th grader (lower), so that is the only perspective I can speak from. I cannot recommend it highly enough. I don’t doubt that entering as a freshman has its advantages, but being a new lower at Andover is fantastic. People are pretty friendly. You won’t have social groups handed to you on a platter, but you can become very close with returning lower groups. There are some who stay in new lower groups, which is fine, because new lower groups are really great, too.
I also happen to know that Andover’s acceptance rate for new lowers is a touch higher, but also it is a self-selecting pool, as someone said earlier. The general perception from returners is that the average new lower is slightly ahead, but I don’t know how true that is. Andover admits about 70 new lowers.
Whatever you do for your other schools, I recommend applying to PA as a 10th grader.