I live in MN and the house temps are usually around 70-80 fahrenheits. I'm going to keep the aquarium away from windows so that there is no change in temps.
I think the 200W would still be good, but also check the size/length because some of them are pretty long. If you choose a 150W, I would get 2 so you can use both during winter months. I usually have a couple of extra on hand to use as backup or to warm up a lot of water.