Get the 3DS XL but don't trade in your Vita. You never know when you might want another one. The Vita2000 doesn't have dat OLED, just LED and it's a noticeable difference.
What you should do is sign up for PS+. If you have a PS3 you'll get games for that as well. The reason I say this is you make mention that you're interested in Mario Kart 7. Sonic & Racing All Star Transformed is one of the free Vita games this month. You can still get Soul Sacrifice and Hotline Miami. Uncharted, Gravity Rush, and Wipeout 2048 will be available even if you don't sign up now.
*edit*
I have both 3DS and Vita. If there's an exclusive for either one of them that I'm interested in, I'll buy it. If it's a PSVita/3DS game, I'll make my decision on better graphics or 3D effect. Both systems are almost equal when it comes to quality exclusives in my opinion. Just as I think that the 3DS has some overrated games, I think the Vita has some overrated games as well. Killzone Mercenary is a pretty game, but that 78 on Metacritic is way too high. 2/10 in my opinion.