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.
And this is the real reason why many will still opt for a 3DS once the same franchises that were on the DS show up on the 3DS, even at $249. Certainly not because of the 3D, and certainly not because the hardware itself represents a better value by any measure of the word.
As far as your rationale goes, you'd have a tough choice based on the 3 criteria you listed because the battery life will likely be about the same (not different enough to be a key factor), the hardware is the same price, and the basic retail games will also be the same price.
Library of scheduled titles and announced titles should be your best bet.