Is it better to have a slightly big or slightly small bike?
If this is your first bike, go to your local bike store and have them fit you professionally. Then, ask if they have loaner bikes so that you can get a feel for the size of the bike before you invest. Some people prefer bikes on the smaller side and some prefer them a bit larger. You'll get a definite feel for this over time, but I wouldn't recommend learning this by purchasing an uncomfortable bike!

I, for example, generally ride a 56cm road bike. However, I once rode a 56cm Surly Long Haul Trucker that felt too large. Some of that feeling was the components (stem length, in particular), and some of it was the frame geometry. Road bikes are built with a variety of geometries catered to every style of riding. Each bike is as different as each respective cyclist, but riding an uncomfortable bike will ruin riding for you. Your LBS will be able to help a great deal in picking a size and adjusting everything to fit you correctly.

Lots of people ask for definite answers based on their exact leg/inseam/PBH/height-leg-ratio-distance-whatever...but most of it will come down to what fits comfortably.