I installed an Inferno in my 2019 X2 and an under-the-seat heater in my Gen1 Wolvy. I would never install one under the seat if you ride through much water because the heater unit will be soaked in mud water, blow the fuse if it's running and water is up to the fan blades, and then blow steam on your windshield. It's a dumb location IMO unless you never ride in water.
The Inferno heater tucked in left wheel well is much better. I made a small rubber splash guard to keep mud from blasting the heater motor. I think it was kind of a pain making it fit, routing the air ducts, etc. It works, and I'm glad I have it, but I'm not sure I would do it again. Don't expect the defrost vents to blow the amount of hot air that your automobile blows. Also, IMO and others, you really need to also install a heater bypass valve to get good, consistent heat.
Bypass Valve | Inferno Cab Heaters - UTV Heaters, Heater Kits I think it's probably the best choice for a heater.
Massive and others on this forum have some tricks for DIY heaters that capture radiator heat or heat from the engine compartment.