Put in the bigger one if you have room. Heatercraft is the most common. When you do your installation put in ball valve shut offs so you can still run if the heater fails. Plan it so you install drain cocks on the lowest spot on the lines to make winterizing easier.
I spent a lot of time locating mine (2) where they were out of the way and the HotTubes would be most useful. I put mine behind the side panels on the side of the hull below the gunnels. Most put a heater in the drivers footwell.