I find freestanding bathtubs very elegant, but I am not sure if the thermal insulation is good. I could imagine that the bathwater cools down quite quickly there.
Although we don't have a direct comparison, the temperature in both the built-in bathtub of our former rental apartment and in our freestanding bathtub stays equally "well." The wall thickness of our freestanding bathtub is also considerably thicker than that of a built-in bathtub.
To test whether we like the way of bathing, we once simply booked a room with a freestanding bathtub during an upcoming vacation - then you have enough time for a thorough trial.