"great work-effort efficiency on smooth roads, but can decently handle the snow."
I agree with other posters . . . a cyclocross bike fits that criteria. They're a kind of beefed up road bike designed tough for off-roading (wider tires, room for fenders, sometimes disk brakes), that will work well on the road too.
Road bikes with drop handlebars are more efficient on the road because of the position that you sit. Your bars are low, your butt is up. Your back is closer to parallel to the road, so you create less drag. The bars are narrower than flat bars, so your arms create less drag. They're also lighter. If you want to go quickly, this is your ideal choice. Potential issues with road bikes:
- Sometimes you can't fit fenders / rack / wider tires. . . fenders and rack are very important on a commuter bike. Wider tires feel more stable and are
- Road bikes are different to ride . . . you need more flexibility to get into an aerodynamic position and they are not as forgiving of bad technique (fail to lift your ass when going over a bump and you'll feel it more).
- The integrated brake levers that are sold with most road bikes are nearly impossible to work while wearing warm winter gloves/mitts. There just isn't enough room for all the insulation and they get stuck when you're pushing the little levers. When it gets below about -8C (with driving winds) I need heavier gloves than can be used with the levers (and I'm pretty warm in the hands compared to most people).
A winter bike is a little bit of a different can of worms. If they salt the roads heavily in Boston, your winter bike needs to have inexpensive components . . . because they'll all get eaten up by salt. I'm pretty good about bike maintenance in the winter, and wouldn't expect a derailleur to last much more than 6 years on the outside. If you don't carefully grease everything and keep up with lube, your bike might be toast after a single winter. At the least expect to replace cables, cable housings, and chain each winter . . . your cassette might last two winters. An aluminum frame is nice to have for the salt because you don't have to worry as much about rust. Some people like studded tires for winter use, but I find they slow you down too much (11 mile commute each way) . . . so have just learned to handle my bike differently in snow.
You can get a bike that will do both, or run a separate winter bike than you would use in the summer (side benefit is that if one bike goes out of commission or you're trying to learn how to repair it you have a backup to use). Right now you just want to go to a close by gym year round, so my advice would be to get any cheap bike that seems to fit you OK. If by the end of next summer you enjoy it a lot and want to do long rides, pick up a fancier bike for spring/summer/fall use and relegate the cheap bike to winter duties.