My DS library vastly outnumbers my 3DS library, though, and DS games DO look much better on the XL.
I "have" 300 games of DS and 4 games of 3DS. The resolution of 3DS and 3DS XL does not change but the size of pixel is almost double, the pixel scale, in some games it is clear sawtooth (small but noticeable). When more complicated is the game graphically (resident evil...), most noticeable, with very flat or simple games not noticed, the quality of manufacture is more "plastic", comes without charger. I did not checked but searching the internet you will see that many people complain of a loss of sharpness in the color (for the new screen).
Indeed, the lower screen size is lower (in resolution to the Nintendo DS and games are altered .. so all the people who had a DS still keeping it to play 100% to the DS games.
But peace XD, buy everyone you think is best. This is just my opinion ..