Go with the two 250 watts. Both my 66 and 70 gallon tank have two heaters on them... and they need it! I used to have one 300watt hearter on my 66, and during the winter, that thing was constantly on... and my water temps were unstable (one end of the tank was cooler than the other end... all the fish crowded on the warmer side). The main floor furnance is turned down during the night, so I can't rely on ambient room temperatures to help stablize the tank neither. With the kinds of winters Alberta can get, its better safe than sorry... just make sure you get a couple of good quality heaters.
-Richer