It depends whether you live in a hard or soft water area.
It also depends on the heat source for the cylinder, if it is a renewable source (renewable, huh!) then copper gives better heat transfer.
So there is no definitive answer unless you know the application.
NB: The worst cylinders I have come across are made by RM. Had three leakers and still waiting for reimbursement for my lost time for the first one. They can't even get the tappings in the same place on two supposedly identical cylinders. So even their replacement was aggro, even after I'd allocated half a day for swapping it. Total cr*p, nill points.
I've supplied IMI and Albion in the past, but having used Newark Copper Cylinder Co for around 30 years I'd give them twelve out of ten for quality, availability and service!
in an ideal world, stainless steel & high recovery (assuming the boiler has a decent enough capacity to make the most of the recovery). If you go with high recovery I always suggest having the system wired for hot water priority, its only 30mins tinkering with the wiring for someone who knows what they're doing.
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