There are 3 factors in gearing: You'd like a gear that will allow you to pedal as fast as you're able. You need a gear that will allow you to pedal up the steepest hill you're likely to encounter. You'd also like resonable steps between your highest and lowest gears to always keep you in a comfortable RPM range.

Standard road bike double gearing generally uses 53 and 39 tooth chainrings up front. Racers need that but, for people like me, that yields some high gears that I never use and doesn't provide low enough hill climb gears to suit my needs.

Typical road bike triples generally provide a slightly smaller big ring and add a 30 tooth granny ring for climbing the steepest hills. They yield the widest range of gearing so, if you don't know what your needs are, a triple is the safest option.

Compact doubles usually have a 50 tooth big ring and a 34 tooth small ring. They'll provide a high gear almost as high as the racer gearset and a hill climb gear almost as low as a triple. So what's the downside? Remember "reasonable steps to keep you in a comfortable RPM range? Compact cranksets have a bigger gap between the two chainrings. To keep the steps between gears comfortable you generally have to make 2 rear shifts every time that you change front chainrings. If the only time that you have to change front chainrings is at the top or bottom of big climbs, a compact crankset will suit you perfectly. If you find yourself having to make a lot of front shifts on a relatively flat road, you're going to hate the compact.