I've never used an auto-inflate, but read many complaints where some brands had gone off in the garage, on a humid day, etc. They are not supposed to inflate due to casual water (splashing, etc), but the performance is going to depend on the brand. Mustang has a good rep; some of the BPS vests do not. Bargains aren't a good deal.
In a float tube, I'd say a manual is a better choice. In whitewater situations, auto is actually required in some waters.
If you get one, get spare CO2 cannisters and TRY IT OUT! You'd be surprised at what could go wrong.