The # of pixels are the same, so by definition the XL will not have as good of quality as the normal 3DS. But it's not like going from standard def to high def.
To me, it's more about comfort. With the standard 3DS, I play with the 3DS closer to my face. With the XL, I play with my arms more stretched and in a more comfortable position for me.
This is a start at least, but I know $20-25 off was the standard this holiday season. Add that in with all the game sales, you could have easily gotten an XL with a physical copy of Mario Kart for $200 this holiday season.