The shape of the faces of the valves will raise or lower the combustion chamber pressures when flat vs. dished due to reducing or increasing the space of the quench area, and therefore creating an increase in power. Generally, having flaat exhaust valves and dished intake valves will add to the swirl effect on exhaust, theoretically helping the exhaust gasses exit the combusion chamber quicker. Now, unless you've got a head thats ported beyond belief or a racecar making a million and a half horsepower, its really not going to make a major difference either way. It's mainly a factor on very built, ultra high-performance engines.
-Jake