Heavily depends on what you value more. I went with a slim vita for the internal memory to hold my hacks, a regular 32gb memory card to hold my PSP games, and an SD2Vita to hold my Vita games. The Slim in particular is of interest to me because the screen will not burn in over time, unlike the fat one. In 15 years the screen on my Slim will still look like it does today, but with a fat one that would most likely not be the case.
However, OLED looks gorgeous so if you don't have a problem with possibly having to replace it down the line once the burn in becomes an issue than that may be the option for you.
Figure out where you own priorities are and base your purchase around that, all versions have their pros and cons and only you're able to know how much any of those matter to you.
EDIT: Important for OLED is also how you use the device. If your play sessions are generally under an hour at a time the burn in will be very slow. My play sessions are usually longer than that, as I ensure I have time for longer ones before playing, so OLED will deteoriate much faster for me than they do for others.