thicker wood insulates more, is stronger, and lasts longer. it's also heavier. no surprises there. the extra insulation helps keep the cluster warmer over the winter, and might keep them cooler in the summer. less bee energy spent on climate control means they'll consume less honey and leave more for the beekeeper. it's still cedar, so the weight isn't too excessive. and unless you request otherwise, I'll still build the roof out of 3/4" lumber, because it doesn't really serve an insulating purpose and gets real heavy if I use thicker stuff. the price will vary with the thickness you're after: $30 for up to 1", $40 up to 1-1/4", and $50 for up to 1-3/4", supposing it's available.
1 1/2" lumber with heavy, heavy Warré roof