Tough question to answer without more information on what you really, really might do with the bike (like what kind of commute do you have and how often would you do it, how often would you like to ride and on what terrain as you train for sprints, etc), but if you have no interest in riding trails, based on what you gave, I'd say go with a road bike.

Plenty of people take their mountain bikes for Sprints, and even a trace few do Olys with them, so if you were leaning toward a mountain bike because, say, you enjoy trails much more than the road, and your commute is half trail and half road including city, then go for the mtn bike. Otherwise, if you'll never ride trails, go with the road bike. But, if you really do not plan to get into the sport too deeply, where much of your riding might be leisurely strolls with a significant other, who happens to have a mtn bike, then get the mtn bike. If you want to go fast and place well, you must have a road bike.

Oops... sorry. I misunderstood your question in my earlier post. I had thought you were asking about a mtn bike or road bike, not a cyclocross or two bikes.

I agree with Jr. Two bikes. One beater for commuting. One for racing.