Actually, I think Ariel would be a surprise winner. Much like Aquaman, her life under the sea has adapted her to be tougher than the average human, and she did not lose this upon being granted legs (otherwise, she would have been crushed by the pressure, to say nothing of not suffering any sort of decompression by Sebastian and Flounder bringing her to the surface.) She also adapts very well, having picked up walking in a matter of hours. As for Elsa, Ariel would no doubt be adapted to cold temperatures- after all she swims under icebergs wearing nothing more than a seashell bikini.