I got a 4GB because I've done research and really a larger card isn't needed. I have the Rayman Origins demo, two WipEout 2048 updates, Twitter app, and my WipEout game saves on my memory card and I still have like 3.1 GBs left. I consider that very good. No, it's not good if you're the type who wants to store videos and music on your PS Vita. It's also not good if you to buy your games digitally. I suggest you save your money unless you feel like you really need a larger card. If you just plan to have game saves, updates, apps, and a few demos; you will be fine. Trust me.
Let me correct you where I think you're wrong. I do realize the thread is outdated but oh well.
I feel as though the 4GB is enough if you decide to get only physical copies of the games because your memory card will only hold the save data, not the whole game *and* save data. The problem with the 4, is that you underestimate how much you're going to put/download. A couple of songs, large videos and so forth are going to fill it up so fast.
Playstation All-Stars is, as of right now, about 2.2GB. That's more than half my current card. And there's more updates on the way. Certainly don't forget about other downloadable movies or apps or small games on PSN either.
It's a decent starter size but I feel like 16 is best.