Manual, Auto or Hammer
The most popular lifejackets are the auto inflating types. The main advantage being that if you were knocked unconcious and thrown overboard the lifejacket would inflate and turn you over.
Hammer lifejackets will also automatically inflate but will not be activated unless the jacket is a few inches below the water. The advantage is that they are unlikely to be triggered by general foul weather or damp storage conditions - factors that can trigger normal auto lifejackets. The disadvantage is that the re-arming packs are rather expensive.
One factor against an auto lifejacket is the possibility of the jacket inflating during a capsize while the wearer is below decks. This could trap the wearer below. It has been the cause of several deaths. If you feel the chance of being knocked unconcious is very low then choosing a manual jacket may be the best option.