I've got an original brick and an original GBA SP. The brick is comfortable to hold, but the screen is tough on the eyes in a world where I've become accustomed to bright, high-contrast displays on everything. I keep it mostly just as a nostalgia piece that maybe I'll show my daughter when she's old enough.
The SP is OK... screen looks nice enough for me, but I feel like it was designed with toddler-sized hands in mind. If I'm playing any game that requires rapid button pressing, it's useless because the screen shakes and I can't see what I'm doing.
Probably my biggest gripe with the SP is the fact it doesn't have a headphone jack. This is a completely ridiculous omission on a handheld gaming device. The only person who can even see the game is the person holding the device, so why would anybody else want/need to hear the noise? It boggles the mind...
For a long time I've been meaning to offload the SP and find an original GBA... this thread is motivating me to get on it.
If the SP is backlit (model number on the back has 101 in it) keep it. Original GBA has an even worse screen than the old brick of an original GB, but the 101 screen can be transplanted into it with a bit of work to give you the best screen and best form factor in one.
Quick rundown of the lighting mods available for Gameboys:
Gameboy - backlight mod available.
Gameboy Pocket - backlight mod available.
Gameboy Light - no backlight mod needed.
Gameboy Color - Frontlight and backlight mods available, backlight mod still clunky and hacky (I think you use an AGS-101 screen and have to cut up the case to make it fit.)
Gameboy Advance - Frontlight and backlight mods available (backlight involves retrofitting AGS-101 GBA SP screen.) Very popular to mod because they're cheap, plentiful, and many people prefer its form factor over the SP.
Gameboy Advance SP AGS-001 - the original frontlit version.
Gameboy Advance SP AGS-101 - "brighter" version with backlit screen. Mainly used as source for screens for GBA mod.
Gameboy Micro - totally cute and small but nobody mods them. Backlit but only plays GBA games.
Nintendo DS: Some people literally cut these in half and turn them into a "Gameboy Macro." I'm not kidding. Basically a second way to get a backlit GBA with the horizontal form factor. (But it won't do original GB games so it's getting away from what we want...)