Declaring commercial interest at the onset:
Std of build and finish, best on the Diadema;
Biggest boiler, on the Domobar.
Most expensive.....probably the Domobar?
Cheapest.....probably the Expobar?
FWIW the Diadema is an excellent machine, with the manufacturer always evaluating and improving over time with R & D. Components are best available at the current time for a machine this size, eg Gicar control box, touch pad and water turbine, Campini SSR / pressurestat, probably an ULka pump but that doesnt matter so much as they all work long term, and seldom fail. Probably the best drip tray in its class, and an anodised corrosion proofed steel chassis not just painted.
All will make good coffee and perform properly, so that isnt really a criteria for selection.......
Suggest not to let others cloud your judgement wrt electronic/ touch pad operation (do a search and you will find people say they are "bad" because they are "automatic". The only thing the electronics do in these machines is give you an automatic cup level cut out.......if you wish to use it. If you dont, you simply use it as you would any other machine, by first initiating the flow through the group by pressing a button, and then pressing the same or any other button to STOP the flow whenever you like. ie you can override the auto cut off function for each and every coffee if you wish (but then you may as well just buy a semi auto instead and save some money).
The electronics in modern machines are quite reliable and seldom fail (especially in low volume use) if reliability is a factor you wish to consider. It will be just as easy or even easier for a toggle or rocker switch to fail on a non electronic / volumetric machine, as a touch pad or control board on a volumetric, and even lever machines have control boards internally.
What I am saying is, if you want a volumetric / electronic touch pad machine, there really is no reason why you shouldnt have one if that is what you want.
Most of our volumetric / electronic Diadema Junior machine sales, go to small offices where the through put will be more than in home use.
Hope that helps.