T B s resident plumber here with over 40 years in the trade. Time to babble a bit.
The under counter units are called "Insta Hot "units. That is actually the trade name for the ones made by Insinkerator (the garbage disposal folks) but they are universally called insta hot units no matter who makes them by the geezers like me in the trades. They work well and are not terribly hard to install. They do BTW have a small tank as part of the installation that goes under the kitchen counter where they are installed. Parts are available for them but I have not had to run any service calls on them. As far as I am concerned ,this is a huge plus for the things. If I don't have to go fix or replace them , they must be working O K. I don't personally have one.
As for the tankless house systems , I don't have a lot of experience with them. I have installed a few on new homes that I have done the plumbing systems in at the request of the owners. A couple have failed on start up and for me , this is a big red flag. The controls are more complex than on a standard water heater and they require more maintenance than standard heaters. This is enough to keep me away from them. On the plus side , they are a bit more efficient than a standard unit and I believe they are a bit cheaper to run. I can't comment on cost as all the ones I have installed have been paid for by the owners so I have never actually paid for one. I am told that they are a good bit more expensive than a standard unit and I don't know if that can be made up for by the lower cost of running the unit.
My personal preference in water heater leads to the simplest type you can get. The more complicated they are , the more stuff there is to go wrong on then and the more persnickety they seem to be. I'm not a fan of super high efficiency units. Again , just more stuff to go wrong in my opinion. A standard 50 gallon HWH is usually my recommendation and is what I have in my house. If you have a huge family , maybe go up to a 75 gal. unit. Some of the houses we are doing are putting in two 50 gal. units in parallel. A bit of overkill in my opinion but they sure are going to have a hard time running out of hot water in those homes. At any rate , all the HWHs these days are pretty well insulated and as such , pretty efficient.
My personal opinion...... we had four people in my house for years (Seahag , Bearbait , Gatorbait and my self) and a 50 gal elec. unit handled the task just fine. Just me and Seahag these days as the two kids have both been eaten by predators.
Oh yeah , my 50 gal unit in the house was here when we bought the house 17 years ago and looked like it had been here for some time before that. It could blow any day now or it could last another ten years. Well , now that I have totally jinxed my self , I guess I should start lining up my next HWH.
