I think either a cross or a touring bike makes sense for what you want to do with it. They may be a few pounds heavier that a racing bike, but you should be able to mount fenders and racks for your commute. Touring bikes in particular have low gears, you can get fairly low gears on the 'cross bike, to get you over the hills. You'll get pretty decent rolling resistance (compared to MTB), and you can go to the drops for wind resistance, on either. On weekends you can go for rides.

If you're want, you can get an extra set of wheels and light tires to save for the weekend rides. I don't -- I figure I ride a bike made for loaded touring, because when I get on, the bike IS loaded! IIRC, all my century rides except maybe one were on touring bikes; for some reason, I like them better after five hours than my lighter bike.