Old GBA and GBASP are the same with respect to what they do.
With $30 more you get:
Backlight (worth this price alone because you're gonna need a light for the old SP, and decent ones run $10 or so)
Rechargeable battery (pays for itself, quick to recharge, recharger is compact - original GBA uses two AA batteries)
More compact, screen-protecting folding form (self-explanatory - you don't need to haul around a million different accessories
The SP is worth every single penny more. I was at Walmart on Friday and they were selling the SP for $85 - I'm not sure if the sale is still going on, though. Good luck.
You might also want to pick up a protective case for like 10 bucks while you're at it, too.
Edit: the Platinum and Flame colors scratch easily, from what I hear. I own a Cobalt and have for around a year, since they came out - it has the crap scuffed out of parts of it. You can barely see them when viewing the unit casually, though.
GBA Rocks.