3 watts per gallon is the rule that sticks in my mind. I also gauge this against the average temperature of the room I am going to keep it in. Water is a great thermal mass that tends to absorb heat from a room that is warmer than the water and radiates heat to a room that is colder, so if the room is significantly colder than desired water temp. you might need more watts per gallon or it could be the other way around. I don't have a preferred brand, but I do prefer inline heaters. Keeps clutter out of the tank and if using glass heaters the fish wont break it.