As an approximation, AC Power = Volts x Amperes. There are other factors (phase, peak vs. average current, sine purity, etc.) that come into play, which is why newer PSU's have power factor correction.
The only requirements are that the cord has to be rated at or higher than the local voltage, that it can handle the current load of the device and that it is long enough to reach from the device to the wall socket.
Applying this to the lower rated of your two power cords, 120 Volts x 10 Amperes = 1200 Watts, which is a sufficiently greater than your 300 Watt PSU that, even allowing for a lot of inefficiency, it shouldn't make any difference at all. If you're the least bit unsure, you can use the higher rated cord with your computer and the lower rated one with a device known to use less power.