I grew up with the Game Gear first, so I'm not being biased, but I'd have to say Game Boy just had more great games. Let's compare:
Game Gear had:
Some good Sonic games
Some great Power Rangers games
BattleToads
And a sucky Mega Man game
Game Boy had:
Super Mario Land games
A bunch of GREAT Mega Man games
A Zelda Game
Pokemon
BattleToads
An extremely long life
Good sound
Kirby games
A Donkey Kong remake
And a Metroid game
But there's always one thing the Game Gear will have over the Game Boy, a minor movie roll!