If I was trying to be competitive racing on road, I'd have a bike for each discipline. That said, the CX bike is going to be much more versatile, and still quite fast. The lower gearing means you'll either be freewheeling more and have a slower top speed than if you had a road bike, but it can still move. It makes hills easier, and will really help you get a smooth and even pedal stroke.
Converting a road bike for CX use entails major metalwork, whereas converting a CX to road use means a slick set of sneakers. You can go farther, but it's really not worth it. You can buy a good used bike for less. For a one bike stable, the CX bike wins hands down.