I love the nickname
Callie, and I love the Greek history behind
Callista. Also,
Estella has much more of an old-lady feel to me, and
Callista has a lot of spunk, having never been super-popular. It's more lively-sounding and I think it's a name that would turn heads in a good way.
Estella seems to be one of those rising celebrity names... or maybe that's just
Stella. Either way, that's holding me off on it.