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Choosing between an electric shower and a mixer shower is not really a matter of preference; it comes down to what the property's existing hot water system can actually deliver, and the two options make very different demands on it.
The heating element is the part doing the actual work, raising the temperature of the water as it passes through the unit, and it is also the part most exposed to hard water. In areas with harder mains supply, elements pick up scale over time in exactly the way any heating element does, which reduces efficiency and eventually causes failure, showing up as a shower that runs cold, runs lukewarm, or trips out shortly after being switched on.
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.
Plumb2U carries the parts and spares for both instantaneous and storage electric water heaters, from elements and thermostats to the fittings that connect them into a system, at trade prices with next-day despatch across the UK. For merchants and installers working from a plumbers merchants account rather than a shop counter, that means the right spare is one order away, not a special one.
For a trade buyer, the practical upshot is that unvented spares 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.
Neither option is universally better. A trade buyer fitting out a small annexe with one shower and one basin is usually better served by instantaneous. A buyer working on a house with a family's worth of daily hot water demand is usually better served by a cylinder sized correctly for the job. Getting the sizing wrong in either direction shows up fast, as lukewarm water, long waits, or an immersion element working harder than it should.