I may eventually get both, but the 3DS won't be for awhile (if ever). I will most likely pick up Vita at launch.
Here is my rationale:
1. Similar Battery Life
2. Similar Console Price
3. Similar Game Prices
It comes down to preference. Would you prefer 3D with Wii visuals or the near PS3 graphics? I prefer the better graphics right now since my PSP already has near Wii visuals.