I actually own both and enjoy both so please don't think this is me bashing one or the other. I do however want to hear people's predictions on this though :)
Things to Note (Or in my opinion)
-WiiU has a brighter future in current Software Lineup
-Vita had a headstart
-Both Sell similar to eachother on a weekly basis
-Vita is being treated as a high end accessory to the PS4
-WiiU is supported by extremely popular Nintendo Ip's and Mascots.
Predictions for Vita to Reach 10m First
Tbone51
JoeTheBro
aryu
Blowoverking
BHR-3
curl-6
aerys
yvanjean
WonkTonodi
Predictions for WiiU to Reach 10m First
AnimeGaming
Jizz_Beard_The_Pirat
Busted
TheGhosts
busbfran
Damian.W
Kaiwaix33
007BondAgent
tripenfall
locknuts
STRYKIE
EricFabien
Archbrix
tiffac
sundin13