For whole house water heaters choose Rinnai. They make some of the best on demand tankless heaters around. There are others for less, but once you get into the install side of the systems you will find that their models tend to have all the extra stuff you need to use that does not come with the other units. Resulting in more labor costs and not as a neat install package once done.
These units start around $1,400 and go up from there. Plus installation. I just installed one and the project price installed was a little over $3,300.00
But with a tank style system at only around 45% efficient added to the standby loss as it maintains the water temp all day until you use it. Compared to 92% and up depending on model used. You effectively reduce your hot water costs about 50% overall.
The instant hot for under sink is simply a plug in unit that is connected to a separate water spout that is normally installed in the hand sprayer hole in the sink.
You can seek out plumbers in your area by doing a search at the top of the page where it says Search Posts. Use the drop down list to find a contractor in your area.
Aames is a great plumbing company, they also give free quotes, have payment plans and come out within hours of calling them
Insinkerator is the best model instant hot water I've seen, have one in main kitchen and one in "cave", you can buy the whole setup at Lowes or Homedepot. Make sure you get the one w/the filtration canister. If anything goes wrong they come out and replace w/the quickness.
I agree with Woodbridge that Rinnai is a great tankless and with Faidra that Insinkerator makes a great insta-hot. We are moving to new offices a few miles away from our current location and both items have already been installed there, that's how much I agree. But you need to get what is right for you for price point and functionality.
There are ways to check out the good companies. The good plumbers will come to your home and talk through all of the options with you without applying sales pressure. They will explain the pros and cons of each make and model. And, they will have gone to training at each manufacturer's location so that they can both install and maintain/repair all of the major brands.
Some companies in ATL don't bother to attend all the trainings. They will install any brand of Tankless water heaters, then you are on your own if problems develop. Check out their training on - and knowledge of - the different products. Be sure a licensed, bonded, and insured employee will be doing the install, and that they warranty their work, and you should be OK.
Depending on your particular requirements and the installation requirements for you home, you should be able to convert your home from a tanked water heater to a tankless water heater for somewhere between $3,000 and $3,500 here in the Atlanta area. Your savings will come from reduced utility bills and the fact that the tankless will last so much longer before it needs to be replaced.
You can find out more at my website: www.atlantaplumbingplus.com, or call me at 770-447-0374 if you just want to talk things through a little more.