Gravity gun hands down. Comparing them is like comparing the games they come from. No contest.
Portal Gun: Has a heck of a lot of charm and originality, plus some occasional humour (though nowhere near as much as people like to say), but when all is said and done, its all over before you know it.
Gravity Gun: Exciting and always useful, and it has practically usurped the crowbar as the prime weapon of the game. Its a star, and it sticks by you for the long hours of playing through the game and its episodes. Also not forgetting the fact that it KILLS people, and when its supercharged it kills A LOT!