XL looks and feels cheaper to me as compared to my 3DS. That said I prefer the 3D on the XL as I find there is more of a sweet spot before it makes my eyes go googly. I'm guessing it's tied to the screen being larger.
I do prefer the sound on the 3DS, but was unaware it could be "fixed" by a...
Have it for Wii U and my 4 year daughter can finish in the top 4 when set to easy. 9 and 8 yo sons can win. It's actually a fun game. As someone else said, it's not Mario Kart, but worth a pickup to me for $22. I think I paid $30 and am happy with the purchase. Also played the 3ds version...