You'd be better with a 200w to be honest.
The only probs with using a 300w is that it will heat the water up quicker which could stress out and affect the fish if there is a rapid change in temperature, also if a problem arises such as the thermostat getting stuck in the 'on' position (which can happen) then a 300w would boil your fish alot quicker and give you less time to react to any changes.
You could try it in the tank and keep an eye out on the fish to ensure they aren't showing any effects of stress, also if you make sure you temperature match your water when doing water changes then you shouldn't experience any rapid temperature changes.
Andy