This is hardly scientific. In fact, it's not scientific at all. But since the 3DS has better screens, then DS games should look better on them, no?
That's exactly what I tried to find out! I'm comparing how the New Super Mario Bros. looks on the DSi XL and how it looks on the 3DS.
There are several things to contend with. Obviously, the DSi XL has a bigger screen, but it is only capable of 16-bit colour. The 3DS has a smaller screen, but a higher pixel count and is capable of showing more colour.
So even if the difference is slight (it is), the 3DS should give your DS games an oh-so subtle coat of new paint. Good news to those who are looking for games to hold them over while they wait for the bigger, yet unreleased 3DS title.