Both are good to go in December, If I were you I would choose Sri Lanka to start with, and since you can plan for Bali till March, you can do it by March also. Find out all about Bali and Sri Lanka below, that will help you make the choice.
Hey there! As a frequent traveler who has visited both these locations I would highly recommend Sri Lanka if you are going on December. Sri Lanka is an absolutely beautiful country to visit for a lifetime experience. I traveled with my family last year to this amazing country with very friendly and helpful people. Our first journey was to the down south of Sri Lanka, where we stayed in a hotel near the beach called Cape Weligama (I'll leave the link below in case anyone is interested). We got a Ocean Pool Villa which had the best views of the ocean and it was a breath taking on the evening hours at sunset. This location has an ideal beach for surfing where the hotel itself provided lessons for beginners. If you are looking for holiday of relaxation this is the best hotel to visit as it provides an authentic Ayurveda experience and has different packages too. This was just the beginning of our trip in Sri Lanka. We then moved on to travel Ella, Kandy, Yala and many more places. http://www.resplendentceylon.com/capeweligama/
I would suggest Sri Lanka as I have recently been there. Its cheaper than Bali and as beautiful as Bali. You get amazing beaches, National Parks, food and good weather!