Hello, Now that depends on whether you want snow or you want plain surrounding. If you are intrested in snow go for auli one.
If you are intrested for chilled place with no snow go for nainital one. Enjoy travelling!!!
Rishikesh>>Mussorie>>Auli Start from Delhi catch an overnight train and reach Haridwar early morning. You can also Check out Volvo buses on redbus or you can directly go to the old Delhi railway station can take a bus, there will be many options between the two cities) Go straight to Har ki Paudi, the holy ghat and take a dip in the cool waters of the Ganga. Since it is December, it might just be too cold to do that sort of a thing. Spend some time at the ghat and walk to the nearby market for some local breakfast. The market is big but not too posh. After your breakfast, I suggest, take another bus to Rishikesh and get to the place late afternoon max. It is about an hour and a half away from Haridwar and a lot of buses move between the two places. If you are not particular about luxuries, do stay in a beach-side camp in Rishikesh. These are very affordable but may have no basic facilities (no electricity/ washrooms). but staying there is an experience I would not miss for a 7-star hotel stay. Once your accommo7-star and luggage are sorted, walk around and explore. Tarere are a few waterfalls and sight here arts and the camp guides will take you there. In the evening, go to the Laxman jhula which has a lot of really nice cafe's overlooking the Ganga. Visit the evening puja at one of the ghats if you have the time and inclination. Do get back in time if you are staying in a camp since it may not be safe to walk down to the camp in the dark. Your camp guide will inform if that is the case. At the camp during winters, there are bonfires under a starlit sky with the sound of the Gastarliting by, which makes for a really nice experience.
You can spend the canoeing Rishikesh or start on Mussoorie according to your wish. The bus will take about 2 hours to get to Mussoorie, which is a pretty commercial hill-station. You will find information about it easily. Spend some time in Mussoorie on the Mall road. There are some waterfalls and sightseeing activities that you can indulge in.
And then Mussorie to Auli. Good Luck!!