The best SP is the AS-101 model as it has a much brighter screen, but few were made as they were made right as the GBA started to die.
Though some feel the SP is damn uncomfy on the hands (and lack of batteries sucks if you don't have a plug nearby).
The screen is smaller and the li'l portable must recharge like the SP, but damn is it easy to carry around.
The GBA is backwards compatible with all the previous GB titles.
If you want something small, bright, and easy on the pocket, get an SP, It's backlit and has a rechargeable battery unlike the regular one.
There are third-party grip attachments you can use to hold it without your fingers going numb.
If you feel you may not like it, get the original model (the one without a clamshell) and a lamp or one of those crazy ass back-lit hackjobs.
The hardware, designed in 1992, was codenamed "Iron Man" and was supposed to be implemented in a new console to replace the PC-Engine after it was developed.