Depends on how hot you want your tank to be ... and how cold your room is!
I have the 300W & it works fine to keep the tank temp at 75F. <I have a total of 90-100 gal
For a sub-tropical tank, 300W would be fine, but if you are going for tropical temperature, you are better off having 2 heaters in that size tank. <2 x 250W if your house temp is average (70F).> Having 2 heaters (at opposite ends of the tank) gives better & more even heating in a big tank, and also if one heater fails, your temp will not totally crash.