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You don't say if the four installers all proposed the same product, or different products. Both methods you mention are in common use, but each is used with a specific type of panel. So which one's the best depends on the type of panel being installed. I suggest you ask each of the installers to show you other installs they did 2-3 years ago that use the panels and techniques they propose for your roof. Then, talk to the homeowners and see if they are happy with the results. Since about 80% of the final quality of the roof depends on the installer, you want to make sure you have the right people putting on your roof. Good Luck.