Ever since before No$ took a turn for DS emulation, VBA had better compatibility for GBA games, a better savestate, recently-played games management and full-screen support. It also lacks No$'s annoying fog transperancy bug. With VBALink, it's easier to link up two different games. No$ has also had issues running DS games after setting a GBA game to full-screen.
All in all, there is no area in GBA emulation where No$ could top VBA, while it falls short on some occasions. While No$ (with No$Zoomer) is the best DS-emulator up-to-date, I'd recommend VBA over it for GBA emulation any day.
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No$GBA Quick FAQ:
Q1: Savegame doesn't load.
A: Set correct save type. Look
here, divide by 8!
Q2: "Cannot Read Data" error
A: See above!
Q3: Snapshots?
A: Only work for GBA games.
Q4: NDS linking, NDS with GBA slot, Picto-Chat, trading, ...
A: Not yet available.
Q5: When do we get a new version?
A: We don't know. Martin won't tell.
Q6: Donation down?
A: Yes, and we can't do anything about it either.
Q7: Game stops without appearent cause.
A: Turn the Sound option on.
Q8: No$Zoomer fixes don't work.
A: They do only with 2.6a.