I have played thousands of hours across the various Nintendo handheld systems and the GBA SP may be my favorite of all. Some complain about the shape, but I played an entire 162 game baseball season on one in a two month period where I had a job that involved very little work and I never had any problems.
The Micro actually makes the lower part of my hands begin to numb after a little while, no other handheld ever has and it may be because I have large hands.
In my opinion a GBA SP and a New 3DS XL are must own items, allows for play of all handheld Nintendo games and both systems are great.
Regarding the GBA SP I use, it is the launch model blue (cobalt?) system. I have the one with the brighter screen as well, but I typically use the one I got at launch for nostalgia purposes. Battery works well still too! My launch DS is the only system I ever needed to change the battery, it started running for less than 60 minutes fully charged at one point.