Neither option is a universal upgrade over the other; each solves a different starting condition, and the right call is decided by the pipework already in the property, not by brand preference.
A thermal cut-out is a safety component, designed to break the circuit if the unit overheats for any reason, whether that is scale build-up, restricted flow, or a fault elsewhere in the unit. A cut-out that has tripped and stayed tripped, or one that trips repeatedly, is a common reason a shower stops working even though the heating element itself is fine. Trade buyers restocking cut-outs are usually replacing a component that has done exactly the job it was designed to do.
A rubber flexible hose is typically a single-layer or reinforced rubber tube, sometimes with a fabric or wire reinforcement moulded inside rather than wrapped around it. It is generally the cheaper option, flexible enough to route around awkward bends, and perfectly serviceable for lower-pressure, lower-duty applications. Its weak point is the outer surface: rubber degrades under UV exposure, ages with heat cycling, and can perish or crack over time in a way that is not always obvious from the outside until it fails.
Plumb2U stocks flexible hoses across the bore sizes, lengths and pressure ratings trade buyers actually need, all carrying the approval marks that matter, at trade prices with next-day despatch. Picking the right connector first time, rather than the closest one to hand, is a small decision that avoids a much bigger callback later.
For a trade buyer, the practical upshot is that unvented plumbing spares supplier are not interchangeable with vented ones. A pressure relief valve rated for an unvented system is a specific, tested part, not a generic valve that happens to fit the thread. The same goes for the expansion vessel, sized to the cylinder's stored volume, and the tundish and discharge pipework that safety fittings rely on to work as intended. Buying the wrong-rated part to save time on a job is a false economy at best.
Signs worth watching for on a job include a shower or tap that has visibly lost pressure without any obvious blockage, a heating element or immersion that keeps tripping or under-performing, white scale flaking off fittings when they are removed, and appliances needing more frequent descaling or repair than would be expected. Individually, any one of these could have another cause. Together, in a property in a known hard-water area, they point fairly clearly at scale.