That's a tough one -- after I bought my first FSR bike, I kept my classic Stumpjumper HT and converted it for off-season road riding, then never rode it (I hate riding on the road). I finally sold that one when I really needed the cash & space. I recently sold my first FSR to pay for my new custom-built FSR bike. Now I only have one bike and I find myself looking at "back-up" bikes!! I would really like a second bike. I don't want to take my super expensive bike on car trips w/ the kids & I miss it terribly when it is in the shop even for a day or two.

If it has only sentimental (but no practical) value to you, I'd sell it to get some extra cash now, then save up for your off-season cyclocross bike later on. If you can really use it as a back-up, then keep it.