This is just one person's experience.
When I became a DM years ago, the shop for which I worked used 7mm farmer johns for local classes in cool (roughly 60° F) water. I assisted a lot of classes with those, and then I became an instructor and taught classes in those suits. (These were older models than you will get now--they were relatively stiff.) For my personal diving in cool water, I had a 3mm Farmer John.
Then the shop got rid of the 7mm Farmer Johns and went with plain 7mm full suits made of a stretchier material that fit the body better. I thought the 7mm full suits were warmer than the 7mm Farmer Johns, even though they had half the neoprene on the core. At about the same time, I got a good deal on a 5mm full sit for my personal diving, and I immediately decided that it was significantly warmer than my 3mm Farmer John, even though it had less neoprene on the core as well.
I have used a drysuit for cool water diving for years now.