honestly, when it comes down to which to buy, if you don't already have a bunch of gameboy games (GB(C)) and don't plan on playing much, if any, multiplayer GBA games, then get the DS. It will play all of those GBA games, will look better than the SP's front light, and you get to play the newest and best DS games as they come out. oh, and if you don't do connectivity with the GCN (which if you don't have a GBA already you probably never will use that feature)
though if you haven't pre-ordered at this point it really won't matter much until after the end of the year.
as for boktai.. I just blasted everything. I don't have either of those frames yet. I will get those frames and give it another try.. I really did enjoy the game.. it would just always come down to playing it for 20 minutes at night and.. ooops.. well, I'm done for the night.