You actually have a choice between them as you can use a water heater or tankless for radiant floor heating and not just a boiler.
Can you use a tankless water heater for radiant heat.
Because the slab installations will require less than 140 deg f water it is safe to use a water heater to heat the radiant slabs.
A tankless hot water heater is ideal for this application since there is no water to be kept hot at all times.
However it is important to understand that not all of them are suitable for this task.
And i would never recommend the use of a tankless water heater for a hydronic space heating system.
So using the unit for radiant heating generally has little or no effect on the sizing calculation.
Sample temperature curve for an on demand unit.
We will also be installing 2 separate water heaters.
One to supply domestic hot water and a separate water heater to supply the radiant floor.
Though they are sold primarily for domestic hot water tankless heaters are actually well suited for radiant heating which operates at low temperatures compared with for example hot water baseboard.
Water heater for radiant heat water heaters are good for radiant floor heating because they maintain a low temperature.
The system with a tankless water heater will also be much more responsive have unlimited supply of hot water and clean water running through the floor heating pipes without sediments and other mineral build ups.
Tankless water heater can be used to produce radiant heating.