I would stick with Continental - they are both the cheapest and offer a non-stop flight for your convenience. Connecting in DEN
is risky that time of year as they get blizzards sometimes - and even just flurries can cause delays and whatnot. Also, Continental has the best service of any of your non-stop options (the others are Southwest [to Oakland] and United).
I would try to get on the 757, but CO
also flies 737-800s on the route which are also nice. I'd avoid the 737-300 unless it is the only one that fits your schedule. There is nothing wrong with a 737-300, but if you can pick between a 737-300 and a 737-800, the 737-800 is newer, has a more eye-pleasing interior, and has bigger overhead bins.
As for Frontier's fare - someone may clarify, but I've always had the impression that flights were relatively cheap between Christmas and New Years' since people often spend both in the same city, but after New Years' everyone is in a crush to get home so fares tend to be a lot higher that week, so I doubt F9
's fare will go down.