I'd go for foam until they're a lot older, able to swim well, and not panic if they go over. Have done so since 3 with my 2 - now 8 & 9 and still have foam - although will need to change soon to manual gas.
no risk of accidental inflation, solid foam takes more abuse when they drag them on the floor etc...and most importantly less for you as a parent to worry about in the evnbt of MOB. As they'll likely splash about in the water at every opportunity risk of setting of a gas cylinder is always there. Plus foam filled act not only as an additional lay of insulation / wind protection, the collar's a handy pillow when they get tired & need a power nap.