I see that you have double posted this, here and in the Bay Islands section as well.
Utila diving is fairly well similar from dive operation, one to the other.
Before anyone can give you a fair comparison, even if it is apples to oranges, you're going to have to decide and state which end of Tobago you want to enjoy diving. You really can't do both, no matter what some salesmen might propose, unless you spend a week on both ends of Tobago.
Are you looking at Tobago's area of Pigeon Point (South/Caribbean Side) or the North end (Speyside/Atlantic) ? If you really haven't made up your mind on that, this is a secondary question and is premature.
You're going to want to do both Islands in your dive lifetime.