It really comes down to either a DS Lite or DSi XL. It is true that the DS Lite will play GBA games and the DSi XL will not. However, it is also true that the DSi XL will play games specifically designed for the DSi and the DS Lite will not. So you have to choose: either you will be able to play GBA games OR DSi specific games.
It really is a tough decision because there are a lot of good GBA games, and I don't really have a lot of faith in the future of games specifically designed for the DSi. But on the other hand, you might get the DS Lite and then a few months later someone releases a really awesome DSi specific game. Either way you will miss out on something.
I personally would go for the DSi XL because it would be really nice to have a bigger screen and to prevent missing out on possible upcoming games. And, like hotstud19us said, you can always get a Gameboy Micro and knock out all of the GBA games you missed. Then you could sell it and make back most of your money.