I'm guessing that the 5500 represents total square footage, not just the bathrooms. You need to answer the question already asked: what is the open square footage of each bathroom?

If there is no other hydronics in the home now, as it appears by your thoughts of using a tankless system, you would be much better off with an electric radiant system. If you have had problems in the past and are not comfortable using electric, use a low voltage "Floorizwarm" system (depending on the open square footage).

If any damage is done to the element in the future, it is traceable and easily repaired by just removing the tiles above the element and splicing the copper stranded, insulated wire with a butt splice and some heat shrink.

Nick a cable and in a month or so, it's garbage. Some elements, like low voltage, can be repaired. The line voltage systems are difficult, if not impossible to fix.

Line voltage also can't be cut and spliced in the field like low voltage. Needless to say, we stick with the low voltage when it comes to installations. I still sell Danfoss when they want to keep within a budget, but I won't install it.

Their efficiency is the same, 100%. In the truest sense of the term. All of the electricty drawn is converted into heat and is fully utilized in the floor and then by you, without loss.  The difference is in the design of the system, it's abilities and repairability.

Yes, line voltage costs less because there is no transformer and in larger systems, no computer to monitor it. They also come with much longer warranty's (25 years) and can also be used as primary heating when designed correctly.  Where you wouldn't use line voltage cables, under hardwood, i.e., low voltage Zmesh can be installed without worry of being damaged by nails, screws or staples.

Low voltage "Floorizwarm" systems can be cut in the field (by a technician using an optional splice kit) and jumped to heat the seat in a shower or around a Jacuzzi.