I preferred controlling Wallace. This is because I seemed to find the game a bit funnier while playing as Wallace. I'm not sure if the lack of speech restricted the comedy with Gromit a little, or Wallace was just in funnier situations (For example, I found Major Crum hilarious, and you had more interaction with him as Wallace).
This poll does not include a "Haven't controlled them" option because if you will be playing the game in the future, you can vote in the future after you have played it. If you do not expect to play this game, then this poll is not for you. ;)