I have an 11 GPM (gallons per minute) Navien and it rocks! Never have to wait more than 10 sec on hot water even 2 floors up. The GPM is based on how many fixtures\appliances you want to run at the same time. 11 does a good job for about 3-4. Can run the dishwasher, washing machine, and a shower with no issues.
Interesting fact... Taco Bell runs all of their restaurants on 2 tankless hot water heaters (or so I was told by a plumber).
The above is in our current house, but we are currently doing a new build in WA. Because of the new energy standards we couldn't go tankless, and I would have if we could. We had to install a heat pump tank with a recirculation system. I don't know anything about those, other than it's supposed to save money. I hope it doesn't make me miss our tankless.