Depends on the situation. Frags bounce, but stickies cause a bigger explosion. If target is close or somewhere that a frag would definitely miss or bounce off, use a sticky. If target is around a corner or behind an object, use a frag and bounce it right next to them XD.
I managed to stuck someones FOOT, head (easy), weapon (It flew across the map - efter the explosion propelled it, and managed to kill someone, and count as a projectile hit - lol) and the wheel of a Warthog.. It went "Fbptptptpt" along xD.
Haha, that's cute. ^w^
And I like both but stickys always work REALLY good for me... my bro used to call me the "Grenade Master", wtf. IT WAS JUST LUCK THAT I KEPT STICKING HIM HEAPS OF TIMES IN A ROW. D: