Priest is one of the best troop types humans have, so I'm not sure banshee is worth trying to use before you get to the 3-star banshee tech. Banshee and blossom mages are super useful for elves before then, because the elf barracks mage troop is pretty wimpy
Banshee specializes against heavy range units, so if you're up against a lineup that's all or almost entirely heavy range, you can certainly try them and see how they compare to priests, or you could try a combination of priests and banshees and see how that works
It's also worth considering training time in barracks vs training grounds, and whether or not you'd prefer to focus on training priests that are good against heavy range AND heavy melee or make both priests and banshees
My human tournament city only has banshees at 1 star so I haven't played with them at all there, once they get to 2 star I'll probably try them out and see how they compare