The cabin sizes are all the same so it really makes little difference. The ship is large so it is helpful to book a room near an elevator.
As to weather, it very much depends. As you've heard - layers, layers, layers. And do expect to be rained on and promise yourself ahead of time if it rains you will venture outside anyway. You may get lucky, but rain is very prominent, especially in Ketchikan.
Overall, Alaska is WONDERFUL and the Infinity is a beauty. We are sailing on her for our second time in two weeks and can't wait.