I owned a Trek Xo-1 before I upgraded to my Jamis Comet and this is what I can tell you. I upgraded because I found I only did road rides and wanted nicer components.
The difference in average speed between the bikes even though there is a 4-5 pound difference in them (Cross weighs more) is only about .5mph. Now, I struggle with an 18-19mph average but a stronger rider is bound to see a more significant difference. However, for only loosing a half-mile per hour, having the ability to race both is worth the investment especially considering the price of the bike you are looking at.
Some down sizes to the Trek XO-1: It comes with Sora components and they need almost constant adjustments to stay tuned. The wheels suck for road too, but this can easily be fixed by buying a good pair of wheels for road use only. That way you wouldn't have to change tires, just switch wheels.
Other Cross bikes come with better components (105 or ultegra) but be careful too look at the chain ring sizing on the crank. If you are going to ride road, you are going to need a 52 or 53 tooth large ring and a lot of cross bikes only have a 48 tooth ring. The Trek cross bike will come with a triple 30-42-52,which will give the best all-round gearing. Also, make sure your rear cassette is has a large spread such as a 12-26,12-27 and not an 11-21
Also take this into consideration. If you already have a MTB, you can pull the bar ends off it and race Cross with that. That being said, maybe you should get a road bike.
