i used butyl on my toerails because it is neat and easy and there is not any stress on my toerails, but on my handrails i used 4200 (3m) because i wanted a little extra grip. I don't yet have a backing plate (inside handrail or whatever) for my handrails so i wanted a little more adhesion than butyl can provide. So far no leaks.
This is a great time to point out 'what you use is not as important as how you use it"...It is paramount to be sure that the screw/bolt holes are slightly beveled out so that the sealent/adhesive gets pushed down into the hole. i scuff up the gelcoat at the area of adhesion with some 120 grit just for good measure. Heck that stuff is slick! That beveled hole also needs to be cleaned with acetone after you bevel it just for good measure so you get good adhesion. And of course the same can be said for the surface to surface contact between the wood and the deck.
I think rebedding is alot like applying a finish. The surface and joint preperation should be the focus of your time and effort.
Have fun!!!!
Keith