I would always go for an auto inflation lifejacket. If you are unconscious you need it to go off!
When buying their are 2 big differences between the two types of auto jacket. You can get a Hydrostatic firing mechanism or a salt mechanism.
The Salt mechanism works by having a salt tablet that will dissolve when it gets wet, once it has completely dissolved it releases a pin which will fire a hole in the gas cartridge. These types of jacket are cheaper but their is a slight risk that they may go off when getting very wet. In 8 years and thousands of hours I have only had one of these types go off when it should not, It was in a bag with water in the bottom. I have been told about people who have had this type of jacket go off when large amounts of water have hit them. If the top is secured properly this should not be the case I have been hit by waves and not had my'n go off.
The Hydrostatic type (more expensive) works on the water pressure. As you go deeper the pressure increases. This means that you can have a hydrostatic jacket getting wet with ought any risk of it going off. They will only fire if the water pressure reaches the correct amount.
If you set the 2 types of by accident I.e. falling in a salt mechanism will cost anywhere from £7 - £20 to refit yourself whereas a hydrostatic type can cost between £10 and £30 to repack yourself.
Other things to look for on a life jacket. I would always always ensure that their is a light on the jacket. If you have conducted a MOB exercise at night you will know how hard it is to find the person. If their is the slightest chance that you will be out at dusk or night then a light is essential.
A harness is a good idea if you are going to be using the jacket on a yacht as it provides a simple place to clip the jacket to.
A spray hood can be a life saver. If their is any kind of a chop then it is very easy to constantly swallow water. A hood will cut this down and their for prolong life.
Croch straps are also a good feature on a life jacket. These are webbing that links the waist belt back to the waist belt undet you legs. They can be uncomfotable to ware but could save your life. I have one but only ware it sometimes. You can buy these at a later date to the jacket for some modles if you want.
Warring a life jacket is the sensible thing to do how ever good at swimming you are in cold water Olympic swimmers can only manage a few minutes at best as cold shock has sapped their energy. Remember more people drown in a harbour or mariner environment than on the ocean. Best advise ware a life jacket and practice MOB with a drum or a ball.
Finished my essay now