I own both.
Vita: Fantastic device, great graphics, delightful performance. I love my Vita and have been playing a lot of it. If you have PS+ or have played all of the AAA titles on it though, the PS4 will probably have 90% of the upcoming Vita titles available for it (as most are indies / AA / ports). I don't own a PS4 or a TV , so I'm happy with playing smaller releases on my vita. However, that may not be the case for you. That said, the Vita has GREAT game sales quite often, so you might still grab a few games for it quite cheap and enjoy them on the go.
3DS XL: I absolutely love this thing as well. It has tons of positives going for it: backwards compatibility with the DS physical roster (which is huge), a great library (which is quite varied). Dual screens are quite big. The graphics are not as bad as they may seem on Youtube Videos, and first party support by Nintendo is HUGE. On the negative side, most games never go on sale, and the few that do are always the same. The indie / eShop side of it is almost non-existent, so you're stuck with AA and AAA games (with all the positives and negatives of that).
Vita games I'm looking forward to: The Vita can have (and does have) weird games, like Akiba's trip, the ever-fantastic Persona 4 Golden (I haven't finished it), as well as the upcoming Freedom Wars and a slew of indies.
3DS XL games I'm looking forward to: The Pokemon remakes! Pokemon X/Y completely revamped the breeding system allowing for easier breeding of competitive Pokemon. Wonder Trading is fantastic. Wi-Fi battles are stable and tons of fun. All of these mechanics will probably carry over to the upcoming remakes. Also, there's Smash coming to 3DS in October, which looks amazing. If you haven't owned a 3DS before, you also have Zelda Ocarina of Time 3D (the remake of the beloved N64 classic), and of course, Zelda a Link Between Worlds (which is my favorite Zelda game of all time). And of course, you have Donkey Kong Country Returns, New Super Mario Bros 2, Super Mario 3D Land, etc etc etc.
So: If you've played the Vita up-to-date and haven't owned a 3DS XL before, I'd recommend the 3DS if you have the funds to pay up for the near-retail-to-retail price of its games.
If you're behind on Vita titles like Uncharted, P4G, Gravity Rush, Tearaway, Killzone, etc, the Vita might still be a good investment, specially if you're interested in indies on the go.
It's up to you.