In every dog class I've ever attended, we were supposed to teach the verbal and then the hand signal because otherwise the dog gets too focused on the hand signal (as they focus more on body language naturally) and never are 100% with verbal commands. I'm not sure how much truth there is to that, but it's just what we've been taught for years and what we've done.

That being said, I use hand signals at home a lot more than I do in public and when I'm in public, verbal commands are used more often. All in all, I'd say I use both about equally though depending on the situation. When we're at the dog park or somewhere where they'e allowed to run around unleashed, the only choice I have is to use a verbal command unless they're looking at me (in which case I'll use a hand signal).