I heard the 64GB ones are very faulty, and the 32GB are not very reliable too.
I got a 16GB and a 4GB with my Vita (that's still to ship to me xD) and I hope that's enough.
The 64GB cards here would cost as much as a brand new PS Vita if they still sold it xP
To me that's a big problem, as I'm completely focused on physical games and even if I would accept getting the games digitally for it the amount of money spent on memory cards would be insane too haha.
I'll mostly use it for PS1 games (those I'm ok with being digital xD) and some indies crossplatforms I already have, but I really want some of those RPGs, specially the Disgaea games and Trillion, but I definitely need them to be physical xP
Since you're focusing on physical games, smaller indies and PS1 Classics, I guess 16GB is fine. Definitely go for a physical copy of those JRPGs though, since they have lots os voice clips, texts and all, so most of them are way too big to get digitally. I played both Final Fantasy X and Persona 4 Golden, and both of them are in the ~3GB mark. I would imagine the Disgaea and Trillion games are also on the +2GB mark.
And yeah, I heard some bad things about the 64GB Memory Cards ones too, it was one of the reasons I gave up buying it altogether. I couldn't justify the price and they are very faulty... like, look at this. I paid R$ 370 for my Vita, the cheaper memory card is R$ 597. I really can't justify this price lol. It's waaaaaaaay too expensive. I didn't know about the 32GB ones though I thought only the 64 ones were faulty, I'll definitely look into it.