Backlit is "better"... both of mine have been frontlit, and they're great anyway. Here's how to find a backlit:
Backlit Model (AGS-101)

Packaging for the European AGS-101 model, in the colour "Surf Blue".
In September 2005, around the time of the Game Boy Micro's release, Nintendo released an improved version of the Game Boy Advance SP in
North America, featuring a brighter backlit screen instead of the previous version's frontlit screen. The new model can be distinguished by the following features:
- The box states "Now with a BRIGHTER backlit screen!" to distinguish the new model from the older, frontlit models.
- The backlit SP has the Model Number AGS-101 on the label on the bottom of the unit, whereas the original frontlit SP has the Model Number AGS-001[7]
- The switch at the top center of the consoles lower face is now referred to in the manual as the "Brightness Switch" and selects between two levels of brightness, "Normal"(Low) and "Bright"(High) with no off setting. On the frontlit models this switch turned the frontlight on or off only. With the backlight set to "Normal"(Low) the brightness still surpasses the original AGS-001 with the frontlight on.
The North American backlit version comes in three standard colors: "Pearl Blue", "Pearl Pink" and "Graphite" (a grayer version of Onyx Black). There were also two
Toys "R" Us exclusive backlit models; a "
SpongeBob SquarePants" model
[8] and a "Limited Edition
Pikachu" model.
In 2006 the AGS-101 backlit model also saw a very limited release in
Europe[9]. Few models made it to market, likely due to the release of the
Game Boy Advance-compatible
Nintendo DS one year prior. The
European version was released in "Surf Blue"
[10] as well as re-issued "Pink" and "Tribal" editions.
Unlike the North American release the European box does not feature any prominent text to distinguish the backlit models from the older frontlit models. In addition, only the "Surf Blue" color was unique to the AGS-101, the other two colors "Pink" and "Tribal" had already been released as frontlit models - for these reasons it can be very difficult to identify a European backlit SP. Apart from the AGS-101 model number on the base of the unit the only other obvious distinguishing feature of the European backlit model is the large picture of the Game Boy Advance SP featured on the front of the box (The European frontlit models of "Pink" and "Tribal" only feature small pictures of the Game Boy Advance SP on the sides of the box and Flower/Tattoo patterns on the front respectively)
So far the European AGS-101 model has only been reported in Germany, Italy and Spain so may not have been released in all European countries.