Buy one wii u one XL

that depends on your preference la. 2 different thing, portable and home console.

just like you're asking buying 1 ps3+ 1 psv, or 2 psv.

List of co op games for Wii U - http://www.co-optimus.com/system/20/wiiu.html

List of co op games for 3DS - http://www.co-optimus.com/system/17/nintendo-3ds.html

For the 3DS... I believe its not complete..

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.