Ah, I'd like to be able to answer you but I am about two weeks out from getting a roasted maple fatback from Warmoth. I'm rather curious myself and decided to roll the dice since I REALLY like the boatneck I got last year. It took me some time to try the boatneck but I loved it right away. I think I have have a bit of room yet in the hand to try the fatback. My concern is that the more rounded, no shoulder shape of it may be a bridge too far but it may just fill out the hand a bit further and work for me. I did order it with a 1 5/8 nut width figuring fatter and a bit narrower was the answer for me.
One thing that helped me decide was getting my hands on a Nash at a local shop with his boatneck and the shop swears they had a fat back from him at one point that I remember. Nash does not formally do fatbacks but, like Leo's outfit back in the day, one may turn out that way here and there. I liked it too if but remember that one Nash I picked up to play there that had an almighty big piece of timber. I'll PM you when my neck comes in and give you my initial impressions. As said, the boat neck I have is comfortingly solid to grab a hold of and should be VERY solid over time. Tone wise I need to get out and playing with some folks to form an opinion but no complaints from the woodshed so far on tone with chunky necks.