I have been around the plumbing/ hvac industry as a rep for almost 10 years and have a pretty solid understanding of it all.
There is no simple answer, itās really about what fits your desire. If you are going tankless, you better have decent water and not live where the extreme water usage limits have been put in place, they suck with low flow water fixtures. I myself, went to tankless in my last house and I really didnāt like it. Iād go with a tank style every time. They are cheap, easy and the myth that the tankless units are more efficient is complete horse crap. They provide endless hot water, not instant, not consistent, not using less gas. They fire off at 199k btus and use all of that energy until you shut it off, a tank style uses 40k btus, heats it passively and provides plenty of water, though yes it always keeps it warm. I have met many people who found tankless cost them more in gas consumption because of the endless water supply and people were happy to use it. Tankless also takes a lot more maintenance to keep the heat exchanger clear. Especially in a time when everything is moving to low flow fixtures ( tankless doesnāt heat unless itās getting .5gpm flow), Im a fan of sticking with a tank. If you want to go green, grab the hybrid electric style tank, over 100% efficient in relation to energy input.
On the boiler, much of the same applies, but there are a ton of great high efficiency options to work with. The key is how much you want to spend and if venting is something you want to change.