My son has played his Wii and Mario Kart a million times over and I want to surprise him with a Wii U and Mario Kart 8 if I can get it at a decent price.
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Honestly man....the way prices are for wii u right now, id just go for a switch. I know someone(a member here and personal friend of mine) who is selling a switch with mariokart 8 and an extra joycon...case....stand...glass screen protector...and all in original boxes. Excellent shape.
What do you consider a decent price? I really don't know what these are going for, but I may be selling mine soon. Mine is modded and has a bunch of games, but that can all be taken off, and I am pretty sure Mario Kart is one of the games I purchased before the hack. I only buy digital though, so if you are looking for a physical copy, I can't help you.
I should also add, even though I am thinking of selling my Wii U, I agree that it would be better to go with the Switch. It is really a phenomenal console, probably my all time favorite at this point. I normally play Breath of the Wild on the TV, but it was great being able to play it on the plane this weekend.