I still find GBA games comfortable to view and on the DS Lite screen, and the backlight brightness can be turned up even brighter than on the AGS-101 model GBA SP, but ultimately I do prefer the larger screen size (and more balanced color saturation, as Jess mentioned) of the GBA SP.
When modding an original GBA with a 101 screen, the default Brightness will be that of an SP-101 on the lowest Brightness setting. To achieve maximum Brightness using the standard ribbon cable adapter you are required to solder one point from the ribbon adapter to the GBA motherboard. Most people don't bother doing this or can't solder.