It depends on what you want. 3DS has plenty of options, but is gonna be the oldest "new generation" device at the end of the year. This means it will be on the way out in 3 years. The Wii U will probably last another 4 years at least.
You can expect graphical improvements on the Wii U titles as Nintendo hasnt tapped that much into the system capabilities while the 3DS is pretty much already beeing pushed to the brink.
At the end of the day, Nintendo provides different kinds of gaming experiences on portable and hme consoles. i would say, if you are looking at the future and dont mind missing pokemon, the Wii U is where you will see the biggest Nintendo games. If you care too much about pokemon and those smaller off the TV screen experiences then go for the 3DS. Its a decision you should find an answer to on your gaming habits.
With that said... i think ou may be making a mistake getting rid of your Vita. The system is gaining alot of support in Japan (it competes with the PS3 in software sales) and PSVita TV will increase the installed base in the west to make it viable to localise those titles. It will be a niche device, but if you like Japanese games its the way to go. Also because the Vita still has potencial power to be tapped for series moving forward in Japan, while the 3DS is aproaching the brink in terms of power as i shared with you before. It will soon be a repetition of the DS and PSP situation in Japan. Despite what people say, i see the Vita lasting longer than the 3DS.
