The DS form factor isn't really akin to that of the GBA as it is - I played GBA games on the DS and I assure you that it's just not "the same thing" as playing them on the GBA Classic, perhaps like playing them on the SP but I personally don't own one.
The closest you can get to proper GBA emulation on the go is the PSP which has become the dominant emulation powerhouse over the years and is pretty much the most cost-effective method of playing your Retro games at this point. Just grab one of those bad boys pre-owned, softmod it and enjoy the results.
There ain't gonna be anything better than that until either the 3DS or the Vita gets hacked, and before you say "Android", "iOS" or "Windows Phone", I reply "No proper gamepad" to shoot down any arguments that may undermine this simple truth.
Alternatively you can just grab a GBA/GBA SP/GBA Micro/DS/DS Lite and a SLOT-2 flashcart, but then you limit yourself to just the GBA and weaker systems, which defeats the purpose of an extra purchase.