I have two- the Simms/Stearns belt-type CO2 model for streams and pontoons (don't know if they're still made) and a BPS collar-type CO2 for boating. Both are manual. One thing I learned is that the CO2 cartridges aren't allowed to be installed during (air?) shipping; you have to open them up and screw the cartridge in before use. Finally found that out after 5 years of wearing mine. Felt a bit foolish, obviously.
Another thing I learned was that some of the cheaper auto-inflating models can activate on a humid day, with age, or with just a splash of water. Read the reviews carefully and don't scrimp on costs.
Personally, I'm ready for an auto, I think. For a few bucks more it makes sense at my age.