The most important thing you want from any of the many home hot water boilers available is that it provides hot water to your family. It should meet your needs, so the first thing to do is figure out what your needs are.
Tanks use slightly different measurements of water supply to those used by tankless hot water heater. Tanks are rated in maximum demand per hour - that is how many gallons can be drawn from a tank in one hour, starting with a full tank of hot water. Tankless units, since they can produce constant hot water for several hours, are rated by the maximum flow rate they can achieve per minute.
For on demand water heaters you need to decide which hot water demanding units you use simultaneously, add up their flow per minute and buy a water heater that matches. For a tank you need to figure out how much you would use in your busiest hour and go for that.
Tankless gas water heaters are famous for heating large quantities of water (though of course you need to pay for these especially powerful units). By not having to be constantly heating the water, they also offer very good value over their lifetimes, saving money over even the most efficient electric tanks.
The downside of instant hot water heaters is that they cost quite a bit more than a tank to buy outright. You may well make this money back over the lifetime of the unit, so the choice is yours.