As you can see, the platinum is 2% to 3% more efficient. If you think about it, if your computer is drawing 650 watts, that's an extra 13 watts being wasted, producing nothing but heat. It may not be cost effective to get the platinum supply, but it will run cooler and the fan will run slower.
Platinum and Gold only measure efficiency not quality. That being said the Seasonic X series are among the best power supplies on the market. The few reviews of the new Platinum series show them to be even better than the X series. Both are truly excellent power supplies.