Actually, I wouldn't be surprised if it was a choice.
Both the GBP and GBA are $50 now. Alot of games are requiring the GBA as a "controller" so it would make sense to own one.
Also, did anyone find out how the GBP will deliever the GBA content (for example, metroid fusion suit to metroid prime--I should post this in IGN mailbag to see what they say).
I think you're going need a GBA still.