The thing about heavy persian archers is that they're still terrible in melee (unlike bosphorans). Their armor really only helps them out in archer duels because their terrible sidearm hinders them so much in melee. Generally, since you can't put the heavy persians in melee anyways, I just prefer Caucasians, the various steppe archers, or, for versatility, Persian-Archer Spearmen because their spear makes them useful against cavalry.

Bosphorans are always great, but their AOR is just so small...