If your only goal is top speed on a downhill then go for the huge chainring. But understand some caveats:

1. You will lose significant utility of several gears, as the next one or two small cogs will be of little use with the large chainring most of the time.
2. Most of your effort pedaling downhill at high speed will go into overcoming wind resistance. In many cases you can achieve the same increase by merely tucking down all the way and moving your hands inward to a secondary grip.
3. The effort you waste by fighting the wind for another few mph's will not be available for the inevitable uphill.

I have hit 55 mph coasting on a hill that was less than 10%. I can't see a circumstance where a 60/12 gear will be of much utility at all.