Ricky, since you have a hard top I like the top mount rather than the bridge mount. Your mounting base is 6'-8' higher in elevation so your antennae doesn't have to be as long or bulky to achieve the same line-of-sight.
Tommy, my thinking as well (easier to run cable also)...what do you think about the "whipping" motion in seas vs. the "double braced" (ie base mount and hardtop support) of the bridge mount?
If no longer than say 6', I don't believe the issue of whipping is a factor given the rigidity of the antennas of that size. I have them on my CC and they are pretty stable.
I have an 8 ft Digital and their is no wip. The antenna is stiff it has no flex to it.
I would put the antenna on the hardtop if it is a 6 to 8 ft 6dB. If you go to 9dB now you are talking a 16 to 20 ft antenna. That has a lot of support and would not work on a hard top. They are usually supported mid way up.
As long as your hardtop has backing to support the antenna base your good. When I built mine hardtop didn't think about it and was cautioned about there was just a couple layers of glass top and bottom of foam and mounting to it. I was told they would of put blocking in before they glasses for mounting things.
Just a FYI. Mine mount to the frame of my tower (marlin). Like others say 6-8' antenna doesn't have much whip.