Is anyone aware of a comparison table somewhere on the net that
would give dimensions across a range of these things?

I don't know, but I would love to see a comparison site with features and
prices. For some reason, water softeners are often sold by door to door
spivs, so it is very hard just to see the range of stuff available and to
compare features and prices.

I would love a dual cylinder metered ion exchange softener (or possibly a
single cylinder meter + timer microprocessor based that regenerates in the
morning if it thinks it will run out).

Unfortunately, the only one of the former I've found is from a (famous)
company that only sell door to door and require you buy expensive
proprietary salt rather than generic sacks from B&Q (or whereever is
cheapest). Most of the others aren't clear about their regeneration strategy
as the marketing ******** is written for people of sub 20 IQ who just want a
glossy brochure with tits on it.

Christian.