If there is literally nothing but a lost turn of production at stake, the obvious choice is to move onto plains hills. I usually will only move one tile, unless their is something amazing 2 tiles away. Usually the choice is muddied by my position on the river. If the end of the river is 3 tiles away from my settler, i would actually move off a hill into flatland, to make space for the second city. Also, future district placement matters. If moving makes so that i cant make triangles of districts near my cap i usually wont do it.