Reguardless of what you decide to use, this is how the White Lightning 3006 has held up so far.
The first picture is from with the initial installation sometime in 2007, caulking the edges between the Starboard and the sides of the boat.
The second was taken today, 3 years later.
Very little shrinkage, still pliable, little mildew, (likely) fairly easy to remove, such as a hatch cover seal.
(sorry the pictures ain't better)