While we're on the subject.. what is the best installation method to prevent leakage? Whether it be water or dust?
Let me point out a couple of things, though:
First of all, if you plan on taking the shell on and off, having multiple layers of camper seal (of any kind) will make it more difficult to get the camper seated. It's not unusual to have to put the shell on and then slide it around a bit until it "pops" into place and that can often tear up the sealing material.
Second: In my experience, the juncture between the shell and the bed is not terribly prone to water leakage (at least not on any of my shells) particularly since the design of most shells has the shell itself "overhanging" the bed edge by a bit - water would have to be coming in sideways to get in.
Now, dust is a different matter but then, you have to remember that unless you seal the entire bed, there's not much point in sealing the juncture between the cab and the shell. On most of my trucks, the vast, vast majority of the dust seemed to enter through the tailgate opening, which, unlike a car door, does not usually have weather stripping.
Also remember that some beds have vents on the floor that also need to be covered or plugged to prevent dust from getting in.
EDITED TO ADD: If you're asking me for a solution to the dust issue, I don't have one. Dust was one of the primary factors that caused me to change from a pickup with a shell to an SUV. I got tired of having to sleep on a bed of dust every night and there didn't seem to be anything I could do to prevent the dust from getting all over everything in the shell.