Which ZP character should i buy?

spop or star if you're looking for a advantage (smaller pixels), otherwise stick with your GP char.

You gotta choose what char suits you the best. If you like the female chars, get one of them ; if you like male chars go for another male char.
If you're collecting ribbons I'd go for the SPOP and if you've got it already then you could earn another ribbon by having the Navy Seal and an M14EBR.
Choose the char you like the most no matter what others might say about it.

go for spop it was the first female character and it's hotter then the rest of the female characters

Go for the sas, ou cant go wrong with... Oh wait.
Dont buy the sia unless your a male who likes other males.

It's all in the benefits (Ribbons or smaller hitboxes).