depends on what you want to do, where you plan sailing
if there is a risk you are going to get tw***ted by the boom or similar, or you could enter the water unconcious, or get so cold in the water that you loose concious / motor control go inflatable, and auto inflatable at that
if there is a risk you will be getting wet on the boat (say on foredeck) then go foam permamnent bouyancy, as heavy spray has been know to trigger auto inflate lifejackets
if there is a risk that your life jacket may be compromised by sharps or sparks then consider a foam jacket.
Dinghy type vests are fine if you don't think there is risk of loosing concious or getting so cold that you cannot maintain the right way up. There are foam vests that will support you but they are bulky and restrictive on movement... ideal for children and or others who cannot be trusted to use / operate inflatables correctly
As presuming Ed and others have said an inflatable lifejacket should be used with a crotch strap (as Spinlock and other high end makes supply)
if you think you will be doing offshore racing under ISAF OSR regs then it needs to have certain requirements.
a croitch strap is neccesary on an inflatable but not on a foam vest as in a vest the arms hold the bouyancy down around the chest
gain depending on what you do in an ideal world it should also have an integral harness in case you need to be lifted from the boat, or as a hardpoint to attach a lifeline to.
you are doing OSR races you will probably also need a strobe light (if racing overnight / in darkness), and some OSR classifications require a spray hood to cover the head/face and strop / reduce
there may be legal requirmements in your area (some places mandate specific types or makes or only approved devices).
if you do buy an inflatable make certain
you also buy a re arming kit (if the jacket goes off at sea then you should be bale to reset it at sea.
ts periodically examined to make certain that its still ready to work (gas cylinder full and properly fitted, autoinflate (if fitted) armed and ready and correctly fitted) no signs puncture or wear and tear. if using an internal fitting such as a Hammar periodically take out the cylinder and check for corrosion on the cylinder as this can be a wear / abrasion hazard, for this reason some people do not use HAMMAR
after all that, examine the market palce, see whats out there. however Ive used Inflatabvles for many years, there is absolutely no way I'd trust my life to a foam 'dinghy' vest, I find foam vests with collars to bulky so its (autoinflate) infaltables for me. as such I use a Spinlock deckvest. in the past I've had similar from Crewsaver and Paramaris, although with out doubt the Deckvest was the best when I last looked at the market place, purely because it came with everything built in. Thesedays Crewsaver do a variant called 'ergofit'
the Spinlock and CrewsaverErgofit jackets are comfortable (enough) to wear in all heat/wind conditions, granted it won't be as effective as wearing a foam vest as an insulation layer in low temperatures but equally you are not going to overheat in one either
The only negatives I have against the Spinlock are
price.. its kin expensive, but perhaps not so expensive when you consider it also has the crotch strap, hood and strobe included
bulk around the neck, sometimes it does interfere getting on or off the rail if its crowded, but may well be down to my bulk.