Most important is it fits and is comfortable
If it is not comfortable, people won't wear them. For kids, pretty color is great too. This will encourage the kids to wear them. I love the inflatables. They are not bulky, and we can wear them for a whole day if needed and after wearing them for a while, I don’t even know they are there. You can choose auto or manual inflate, and everyone has their preferences. We wear Sospenders with built-in harness, and we also clip ourselvs in when sailing in real rough weather, or only a single person in the cockpit, or at night. We also have 2 type III as backup, and 8 type II horse collars as backup's backup, and for guests.You will encounter people who wants to be macho, and says they don’t need PFDs, or they already know how to swim. Well, let the boom whack them on the side of their heads, and then you can watch them swim, if they are able.There are different types of PFDs. Type I, II, III, IV, V. I have included a link for them.Don't get Horse Collars. Well, get them but keep them as backups. They are cheap, works, but are ugly, hot, and very uncomfortable.Of course, it goes without saying that it should be USCG approved, and keep the person's face above water, which is a must for life saving.