I have an Aqueon Pro in my 29 gal. It hasn't given me any trouble, and I really like how it doesn't stick up as much as some others do. I have it sitting next to my filter intake, and I didn't even have to cut any more from my hood to place it in.
The way you determine the wattage is 1) by tank size and 2) by how many degrees above room temp will the tank be heated. If you have a cold house, you might want the 200w.