I just got a 3DS XL for myself and tried the 3D but it strained my eyes in no time so I turned it off and the screen looks like any other screen, it just happens to be in your hands due to the nature of the system. The bigger screen on the XL is pretty nice as well.
I don't know much about the Wii-U, but I believe it can play Wii games so that opens up a decent sized library. It is nice to play on a big screen but it really comes down to which system you think will provide more/better games. The 3DS has the big DS library as well, which is rather intimidating due to the amount of high quality games available (plus it has Pokemon, which is a plus).