The SF axle shafts are the same thickness as FF shafts where they go into the diff, and then get thicker as they go out into the axle housing. This taper apparently stops them from twisting, making them more prone to snapping. But the FF also have their own problems. Shoddy mechanics like to hammer the edges of the drive flange to release the cone washers, and this can lead to deformation of the axle flange and issues with breaking hub studs down the road. If the bearings aren't regularly serviced they will also wear the axle spindles which are not replaceable.
I replaced my SFA with a FFA because the shafts were scrap, and I think the SFA design is rather poor on the 60 with the wheel bearing rollers acting directly on the shaft itself, plus those C washers as the only thing holding your axle in. The Hilux rear FFA is a better design IMO. That said, if your SFA is in good condition, I would not go to the expense and risk (of getting worn spindles or damaged axle shafts) of replacing with a FFA unless you need it to run a locker or LSD. The SFA has proven itself over the years in the 60 Series and other cruisers, they are not a weak point of the vehicle.