Though I have never ridden a Thorn with a long top tube, I owned a size 560S Sherpa Mk2, my Nomad Mk2 is a 590M and the bike so kindly loaned to me by a Forum member for my 2014 tour was a size 587S Raven Tour.
By adjusting cockpit length via different reach stems and drop handlebars with different reach/drop (in my case, Thorn-supplied Zoom Anatomic vs Bontrager compact) all these bikes were made to fit me within 1mm variance, essentially identical. My preferred back angle of 45° with hands atop the brake hoods and elbows slightly bent for shock absorption is the same as well All were fitted with 26 x 2.0 tires.
Although my contact points on the three bikes were the same, the Nomad's top tube is longest. It gives a feel of riding "in" rather than "on" the bike and I like it very much. My body, butt to hands, is wholly and equally within the wheelbase of the frame and it handles and steers very well.It also handles extreme loads easily the best of the three. While my camping gear is light, my extra weight comes from carrying a lot of food and water for my solo, self-supported desert tours, away from resupply. The bike handles well with or without a load. The Nomad has the shortest stem but the widest handlebars, so the moment arm/distance between my hands on the brake hoods and the center of the steerer is nearly the same as with narrower 'bars and a longer stem so steering and leverage are nearly the same as well.
My Sherpa did not have the Nomad's ultimate luggage capacity, and had the shortest top tube and longest stem of the three. It is a bit hard to compare due to variations in frame geometry including chainstay length and of course weight, but the Sherpa felt nipper in handling and the shorter top tube felt -- in contrast to my Nomad -- as if I were riding atop the bike and the longer reach stem felt more like a tiller.
The Raven Tour slotted in between in terms of feel...again hard to judge in terms of top tube length alone because there are many other variables at play that contribute to the overall feel and "flavor" of the bikes.
Of the three bikes, I appreciate the Nomad's more steeply sloped, lower top tube for the extra standover clearance it provides, of great value for me on deeply pothole roads, logging roads, and cross-country where I sometimes find myself dismounting into a hole in the road surface or ground and clearance becomes less generous.
Best,
Dan.