Sorry this might be long and my technical terminology might not be correct.
We renovated a derelict property which needed all new pipework. We had several companies come to demo their machines and we did alot of reviewing.
From memory there are 2 main types, those with and those without electrics. The electric ones flush/recharge the system at a set time each day- normally middle of the night. The problem with that type though is it will do it, even if you haven't used any water all day- such as when on holiday. It also uses water and electric to recharge it.
We bought the non electric one, which only recharges when it needs it. If you have a large household and don't go away much, then then electric one might be better. For us though, its just 2 of us, but a large house. We do often go away/work away, so the non electric one suited us.
We have a booster pump and hot return, so never noticed a reduction in pressure. In addition to inside, we also installed garden taps at the front and back. To wash the car, attach to a karcher for the windows etc. We also have regular garden taps for watering. In the kitchen we have a 3 way tap. We also have one in our en-suite. It means I can get fresh water at night without coming downstairs. I know the 'salted' water is ok to drink, but I can't stand the taste!
In terms of salt, again, several types. 1 brand, I think Harvey, makes a salt block shaped to fit their machines only. We didn't want to be tied into only being able to buy their own, branded salt. Other brands do a compressed brick shape. We went with the pellets, which are about 1inch square little blocks. You can buy them from nearly every hardware store, builders merchants, plumbing supplies etc and just seemed easier to find and cheaper than the blocks.
Another thing to consider is hopper size- where you put the salt into the machine. Although its only DH and myself, we often have friends/family and when its a full house, some of the salt machines would apparently need filling every few days! This seemed like such a faff. Luckily we had the space, so we have a hotel sized one. Its about 1.2m tall and round. Yes, its excessive, but we only fill it annually and can buy the salt in bulk for a cheaper price. Its a shame they didn't make a size between a normal domestic one and the hotel one though. We went with kinetico brand.