tankless water heater/boiler?

Thanks to everyone who helped out with the direct vent/power vent water heater and boiler question a few days ago.

Now I have another question. I found many sites with tankless water heaters which have very high efficiency. But I am worrying whether this will cause unstable water temperature in practice. Could you point me to some reviews, or share your own experiences with me?

Also, I am not clear what is the difference between a hot water boiler and a water heater. I know that a boiler has a lot more heat capacity (in BTU/hr). Howeve, I don't know whether it has a water tank as well, and whether the size of the tank is important (it is not listed in many specifications). I also appreciate help on that issue.