I am having a T-Top made for my boat on which I will install a
Raymarine radome. Is there any advantage to mounting it on "power mount"
(Seaview) as opposed to mounting it directly on the top, other than
gaining a few inches in height? The top will be canvas with a mounting
plate installed.

How much clearance is there? Enough to let you install/remove the canvas
without the radome interfering? The mounts generally let you get two
things: clearance and down tilt. Speak with your boat maker about how much
tilt is common on your model vessel. You don't want to have the radome's
plane of coverage being so low as to not clear the bow (not very likely but
something to consider). Also check what the total height would end up
being. No sense making the radome higher if it ends up being too low to
clear bridges. Again, not terribly likely but something to consider.

The seaview mounts also have options on mounting the light behind/above the
radome. A nice way to keep the arch from getting overly crowded while
keeping that nav light visible.