Agree with lolix, go ahead and get Spica. She really IS the best non-nanaly archer at the moment. Bashira (the archer you missed) is also a great archer but in terms of raw damage Spica wins as Bashira is more of a burst archer. Spicas skill also lasts for eons and once they get us closer to the japanese version of the game, Spica will gain an even better skill.

So yes, get Spica while she's still here. Chydis might be good, but we have no idea when the TP will change, nor if Chydis will even be in that TP change.