It's been awhile since I've worn a farmer john 7mm, but if comfort dictates that much rubber where you dive, I'd say it's probably a toss up. You'll certainly have the option to stay warmer in the drysuit, but you may well still have lesser mobility and more drag with the drysuit. With enough effort and expense - research, trial and error, custom fitting - you have some control over those negatives as well. I hated the bulk of the old 7mm farmer john but I still recall it being better than my off the rack uncompressed 7mm neoprene drysuit. For warmth the two were not even close, especially once I found out about the 12mm drysuit hoods from Otter Bay!
One negative aspect of drysuit diving I don't often see discussed is control of over-heating during the surface intervals, especially leading up to the first dive. Down here we have water temps that are often 50F or even a little lower, coupled with air temps that can range into the 70s or higher, often with intense sunlight. After switching to drysuit diving, this became a vastly more serious consideration for me. I find it a critical challenge to manage comfortable body temperature once I've got the suit even partially donned. Lack of shade, a long walk to the shore, or even sitting in the sun on a boat, result in a real risk of extreme distress. With the uncompressed neoprene suit I wear only a thin liner and find that with the suit down around my waist, I have some ability to manage comfort for a short period of time in the heat. When I tried a White's shell suit which necessitated two layers of heavy fleece, I was literally on the verge of passing out by the time I got the undergarments fully on, then the suit, then the rig, and into the water. If I dove where air temps were reliably much cooler, it would be a different matter. At any rate, that aspect of difference even between two styles of drysuit was driven home dramatically.
In conditions where I'd probably stroke out, I see folks walking around the parking lot, drysuits fully zipped up, while shuttling their gear from car to boat, apparently perfectly happy, so I'm also left to conclude this is an issue that varies a lot among individuals. I see the same wide comfort range on the ski slopes, and occupy a similar spot at the extreme.
When I can, I still prefer the wetsuit for the much lesser drag and greater limb mobility, and simpler management above and below the water. For slower paced dives, I've made the concession on those counts for the vastly preferable comfort level.
