Seriously, I like 'em both, but onions win out for me.
Now about that pesky clumping: has it ever occurred to you that the last time you sprinkled it from the jar, it was above & into a pot of cooking food?
The steam rising from the cooking food is what makes your garlic or onion powder clump, so to prevent that, never sprinkle it directly into food on the stove-
sprinkle it into a little dish, then toss it into the cooking food.