"Buona sera" is more formal, you can say "ciao" or, if it isn't too late (till 20:00 - 22:00) "buona serata!" (have a nice night), if it is later or you are going to sleep, you can say "buona notte!", also "sogni d'oro" (have golden dreams) if you are greeting someone going to sleep.

Good night -->Buona notte. Good evening-->Buona Sera