The cheapest one available. Water output is limited by the power of the heating element, so it won't make any difference whether it costs £60 or £400.
The £400 ones can be repaired by replacing parts - many of which cost more than the entire £60 shower.
Ultimately they are all made of cheap plastic parts which wear out, and will all be destroyed by limescale after a few years.
Also note that even the most powerful 10kW electric shower is still feeble compared to a combi boiler which is typically 30kW or more.
We have had triton showers for many years and their customer service is very good in my opinion. As to your question about output , a lot depends on the mains water pressure to your house.
Our last house had poor water pressure so we had to have the shower connected to the water tank in the loft and a pump to give us a good enough shower.
Our present house has good water pressure so it is connected directly to the mains water supply and the output is fine. You will rarely get a shower that is like a power shower, but it will be sufficient to get a reasonable flow rate.
Get your mains water pressure measured and select a unit that will match that flow rate. Oh, and I would recommend getting a 10w rated shower even if you have to put in a bigger cable because they are better in winter at maintaining a good flow rate at a reasonable temperature.
We also took out one of their service agreements and that has served us well.
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