If RP is your main concern I'd suggest rolling on Balmung (though as I understand that may be full as well?) or waiting for Gilgamesh. If RP is just a secondary or tertiary concern then there are plenty of servers old and new to pick from.
Other than the fact that you'll see people in AF gear or riding Legacy Chocobos on Balmung I actually didn't see that much of a difference in the two servers. The economy seems pretty much the same as of yesterday, and the amount of new players on Balmung vastly outweighs the old ones, so if playing with Legacy players doesn't scare you I'd suggest giving it a try.
Other than the fact that you'll see people in AF gear or riding Legacy Chocobos on Balmung I actually didn't see that much of a difference in the two servers. The economy seems pretty much the same as of yesterday, and the amount of new players on Balmung vastly outweighs the old ones, so if playing with Legacy players doesn't scare you I'd suggest giving it a try.