comfort and protection.
In both the helmet and pfd fit is key.
You want to be comfortable wearing both all day long, as well if your helmet does not fit it will not protect you like it should, and if your pfd is too loose it can come off when you need it most.
Not everbybody is going to like the same fit so try on a few. front zip, side zip , over the head.
If you are paddling alot look for a heavier duty outer material like corduray, as well as I like neoprene lined shoulder straps for better comfort.
In a helmet look at the construction, the helmet is designed to take impact. Some helmets absorb that shock through the shell i.e. shell breaks upon impact.
Others use different foams to absorb, (EPS and EPP)others just use old fashioned non impact absorbing closed cell foam.
Sweet helmets absorb the impact through the shell but in a different way, they use varying angles of glass and carbon to dissipate the shock through out the shell.
Disclaimer- I rep extrasport so I like and use the Pro Creeker