If you have a collection of GBA cartridges and you want to play that, then go for a DS Lite. It's the one reason I can think of, aside from arguably, battery life, or a very low budget. Edit: Or region locked DSi games that can play on the DS.
If you absolutely don't want a 3DS, just get an XL, so far I've heard it has the best battery life, and bigger screens to boot. Flash carts are compatible with the latest firmwares too. It should provide the best experience in playing DS/DSiWare games only with the bigger form factor. The 3DS can squeeze probably 5-6 hours or more with DS games only, and you can future-proof your console in case there's actually a good game you like for the 3DS coming out.
So yeah, in terms of mere choices, go with a 3DS, unless you're on a tight budget or want to play GBA cartridges. The XL and 3DS are only a few $ away from each other, so that's only a purist option.
EDIT: As for the post above this, while it does have a different quality, some don't mind, some do, so it's really up to the TC. It has different effects on 2D and 3D games. I'd personally go for an XL if I didn't want any 3DS games ever but that's not my case.