GBA and DS both have an awesome library of great games, so you wouldn't have to buy either JUST for DoS. Here's a few reccomendations for both:

GBA:
Gunstar Super Heroes
Final Fantasy IV Advance
Shining Force: Ressurection of the Dark Dragon
Mario & Luigi: Superstar Saga
Metroid Fusion
Castlevania series

DS:
New Super Mario Bros.
Final Fantasy III (when it comes out)
Puyo Pop Fever
Kirby: Canvas Curse
Castlevania: Dawn of Sorrows
Pheonix Wright: Ace Attourney
Trauma Center: Under the Knife

Not that you have to go on a spending spree or anything, but they're all good options for your consideration. Now, you have to know a few things about compatibility...

GBA: Plays all GB, GBC, and GBA games, as well as the e-reader.
GBA SP: Plays all GB, GBC and GBA games, but the e-reader doesn't fit.
GB Micro: Only plays GBA games.
DS: Plays DS and GBA games only.
DS Lite: Same as above.

Just keep that in mind before trading anything away.