In absolute terms, Becky, a road bike is the way to go. Its more aerodynamic than a cross bike, and the drop-style handlebars afford you a greater variety of hand positions so that youre more comfortable on long rides.
But life is relative, not absolute. Road bikes have their disadvantages, especially if you havent ridden one before. In short, they can be dreadfully uncomfortable due to the hunched-over riding position. So you have to take that into consideration.
In a cross bike, I like something such as Marins Lucas Valley (US$800; www.marinbikes.com). Its a fast cross bike, with a light aluminum frame and road-style tires. But its also got the slightly upright position youre accustomed to on your mountain bike, and a triple chain ring for hill rides. Also, it can take light touring gear, so you can put on fenders and some racks and spend a few nights on the road. Giants FCR2 (US$650; www.giant-bicycle.com) offers a similar package in a bike thats sized and fitted for women.
You might also look at a touring-style bike. These have road geometry with a slacker frame for a smoother ride and typically offer a little more upright seating than a true road bike. Treks 520 (US$1,200; www.trekbikes.com) is a classic of this type, with a comfortable aluminum, upright stem so youre not crouched over but are still fairly aerodynamic. It has lots of places to attach fenders and racks, and gearing thats well-suited for long rides. Its an excellent all-around bike.
My advice: Test a few different bikes to see what feels right. Try to persuade the shop to let you have five miles or so on the bike, so you can really get a sense of how its going to feel after more than a few minutes.
Youve got your winter gear, now get outside and use it. Away.coms ski and snowboard guide makes it easy to find nearby slopes just begging for fresh tracks.