I have both but i'd recommend the 3DS more if you are on the go.
The list of co-op games in terms of fun is all on the 3DS.(Wii U one are fun as well)
The Wii U games almost more than half are remakes game.
3DS game list are huge while Wii U...not so much.

There are some setbacks if you are getting 2 3DS, one is that you need to buy 2 games to play co-op unless like Luigi's Mansion which has download play.

The cheaper option would be the Wii U as you just need 1 game. All depends on you in the end. We can't really give a good reason which one you should pick.