It does have a headphone socket, you just need a small and cheap adapter cos it's not standard size.
The folding design makes it much more portable, it's much smaller and you have no worries of the screen getting scratched. Also the battery is much better in the SP.
I don't see the advantage of the afterburner myself anyway, you have to pay £24 for the kit, or about £40 if you want it installed too - otherwise you have to take the GBA apart yourself, voiding your warrenty and risking damaging it (if you aint too hot with electronics). That brings it up to the cost of the SP already...
I agree the £50 more is a lot to pay for something that basically does the same thing, but imho the console itself is far superior to the original GBA which a couple of my friends have. Like them, if I had paid £80 for a GBA a while ago I would be pissed off I hadn't just waited for the SP.
btw where did you get a GBA for £34.99? I haven't seen them anywhere that cheap yet (although I aint really been looking!)