Check out the vita thread, this comes up quite regularly. If you plan on being a digital only gamer than 64 gb might make sense. I'm doing fine with a 32 gb I grabbed on Amazon warehouse deals, I also buy a lot,of my games in physical form. You can always delete games and re download them from the psn but that puts a lot of trust that those games will always be there on psn to download.
The past few Black Fridays GameStop has had the 16 gb for ~$20. darby was selling some smaller sized cards if you wanted to start small and see how it goes