Boiler
I think an on-demand unit, which are typically more expensive than a traditional water heater, would provide the greatest savings benefit of not needing to keep a large volume of water hot at any time. It also has no pilot light but in comparison that savings is small.
But, I don't think these are exactlly relevant to a floor heat system. My new boiler, a Knight unit made by Lochinvar, has electronic ignition (no pilot) and only fires when needed to supply floor heat. Of course, once it fires it usually runs all night long until things warm up the next day.
I am really pleased with the Knight unit. I replaced a badly oversized 350k weben jarco boiler with a 105k Knight / Lochinvar unit, and it works great. And, the Knight fires up and runs at 100% flat out until the glycol, piping, and slabs warm up and then it modulates back down to as low as 20% of full bore. After a few cycles it finds a happy medium and then is burning a lot less than it would without this modulation feature.
I put a PC at the wash, connected it to the boiler, and used remote desktop to tinker with it last year. There are many settings you can use to tweak the system and this let me make these fine tuning changes from the warmth and comfort of my home instead of at the wash. But, for that you need the optional PC interface kit.
Chris from carwashboilers.com was very helpful-- giving advice to my local plumber on the installation. The install was quite expensive, but I'm past that now and am very pleased with the result.