UJ rather than flexi shaft if there is any significant load / stiffness.
By the very nature of things flexi shafts tend to wind up a bit under load which will probably give poor feel and make operation imprecise. The one garden machinery flexi shaft I have had anything to do with seemed quite springy and had significant wind up. We concluded that this was a feature not a fault as it would act as a drive line shock absorber. Good idea for a brush cutter or similar where the blade is almost certain to hit something pretty solid several times during the machines life.
Seem to be a decent selection of affordable UJ joints around from various suppliers. Once used a socket set one for a quick'n dirty temporary job. Worked but not particularly wonderful in operation compared to proper UJ joints. Clearly made for strength not smoothness. When you are trying to teach a stubborn nut or bolt manners an extra ft lb or two to drive the joint is pretty inconsequential. But a control needs to be smooth.
Clive