Biggest difference is when you want to remove it.
Jim:I won't use 5200 on my boat. This is for PERMANENT attachment. Things like cleats, tracks, sheet stoppers, deck organizers, winches etc. need to be repaired or replaced from time to time. 5200 is like concrete. 4200 give you all of the qualities of 5200 without the chance of removing your gelcoat with the hardware.As far as I'm concerned I would only use 5200 on something like a bond between the keel and the hull or some other part that was/is not intended to come off. GE, Sikaflex, DAP, Lifecaulk and 3M all make excellent sealants that are just as good as 5200 for their intended use. I realize that Anti-bond is available to 'help' remove 5200, but there is no guarantee that you can get the Anti-bond into the area that needs to be removed.