I teach both. My guys are 100% reliable with both verbal commands and hand signals. It's alot easier getting my boys to do a command when I'm across the field by moving my arm, than it is to yell out a command. They learn that they need to focus at all times too.

It's funny cause sometimes I will give the hand signal for down (arm straight up) while Kai's looking the other way. He'll plop right on down. Amazing peripheral vision

I'm still teaching Keeva some obedience commands.. in the beginning I use just verbal commands (I usually have one hand busy with the leash and the other getting her into the position). After she starts catching on, I introduce the hand signal while still saying the verbal command. Once I'm really sure she understands what I want, I go back and forth until she understands both the hand signal and verbal command.