Depending on your frame bigger rings may not clear the chainstay. Other than that, and other things being equal, i think bigger is better for the reason given by lyledriver. I think as jacobpriest says there is a belief among track racers that smaller rings and cogs are quicker or easier or something than bigger pairings with equivalent mechanical advantage. However, I suspect this is one of the many old wives tales of cycling, ie something that everyone repeats but that probably wouldnt stand up to serious testing.