Well, if you've got a 3DS already, then I advice you don't. There isn't THAT much difference (there is one game exclusive for it which in fact can already be played on the Wii).
The new generation handheld will be out and should be announced soon.
Assuming you already own a Wii/Wii U of course...And assuming you can find the game for a reasonable price in the first place these days.
It's also just one exclusive game so far. That's like assuming Killzone:Shadows Fall and Knack were going to be the only PS4 exclusives ever, or that the N64 was only ever going to have Super Mario 64.