I haven't been to the Maldives (yet), but I've been to Bali a couple of times, and I love it. It's probably on top of my list for where I'd want to spend my honeymoon.
As between the two, I'd say which one will better suit you will depend on what you're looking for. Total escape, peace and quiet, lots of quiet together-couple-time, relaxation-- that's definitely Maldives. If you're looking for a ton of culture and varied natural beauty with all kinds of places to explore and things to experience, that's Bali. You can get the peace and quiet in Bali too, for sure, but (if it's your first time and you're not there for a month) you'll be missing out on lots of things to do and see. Bali is a very big island--certainly bigger than I imagined before I began studying it closely while planning our first visit. The people and culture you find there are not to be missed. So if your plans for the honeymoon include not leaving the room or the resort for extended periods of time, Maldives might be a better fit.
December-January is a peak season in Bali, and I presume it is in Maldives as well. Nothing's gonna be cheap. On the whole, I'd say Bali is somewhat more expensive than Thailand, if that helps provide a comparison.