Both of these 'improvements' are going backwards in my opinion. Now they are selling plastic wheels and locking the machine with their 'posi pin' while sewing. Sailrite used to state (I spoke with the founder when looking to purchase a mini walker) they bought the machines offshore, disassembled, reworked the crude castings (essentially polished them for cosmetics) and re-painted them. 99% of the complaints I read are user caused (but they don't know how to operate the machine or sew and blame the machine) and would happen with any machine. Maybe I'm wrong but I think the 'notoriously bad quality control' is long outdated and more unfounded hype than fact. About 15 yrs ago I bought one of the cheap portable china mini walkers and that's my go to machine now. I sold the shop in 1981 but continued to sew canvas as a boat hobby, however the 95-1 is still with me and has sewn a shxxload of large sailboat awnings (100s of yds of sunbrella with v92 thread). Unless you just want to have the latest and greatest electronic gizmos I wouldn't sweat using the 591 or the old fashioned clutch style for home hobby stuff. Our larger machines used the bigger v138 thread. All the smaller machines handle up to size v92 bonded dac thread (which was standard in the marine canvas trade). During the early yrs our backup machines were home brew metal gear types and an old 1928 industrial Singer 95-1 purchased from a men's tailor shop.short straight stitchs and no reverse but never the less for garments. The 138 was our primary machine for about 2 yrs until we expanded and needed bigger, faster walking foot machines.heavy duty consews and adlers which were not well suited for delicate sail work. It would do everything from curtains to cushion covers to awnings. It wasn't the 'best' for sunbrella or slippery dacron sails but got us through full time sewing quite easily.just at a slower pace. I believe it was actually lighter duty than your 591. As a result of flipping sailboats I started (1975) a marine canvas shop with an industrial pfaff 138 'garment machine' with stand/motor/clutch purchased from a bankrupt sail loft (back in those days the 138 was used in many sail lofts throughout the country). For a home hobby machine the 591 and standard motor/clutch should do boat canvas and sail repair just fine.

I don't consider myself an 'expert' but can speak from my experience.

Singer 591 repair manual