If you prefer to use your Vita the way I do - downloading games including PSP and PS1 classics - then you're going to need a high capacity memory card. At the time of writing this is the highest capacity available in Europe, although do bear in mind there is a 64GB version releasing shortly in Japan which should at some stage make its way over here.
It's just much handier and makes life easier having your own catalogue of games quickly accessible straight from the Vita menu. At £39.99, Amazon seem to be the cheapest going; at time of writing they're having trouble keeping them in stock but hopefully this will improve.
To give you an idea of how useful the size is, I currently have Final Fantasies VII, VIII and IX, Tekkens 1, 2 and 6, Soul Calibur Broken Destiny, and Wipeouts Pure and Pulse saved onto mine, and still have 21GB free which I plan on slowly filling. It feels good knowing you have too much space!