My husband and I will be in Montreal December 7-11, and I have booked a room at the Hotel XIXe Siècle. Is this a good choice in a central location? From what I have read we need to bring plenty of warm clothes. Any tips on "must see and do" while we are there would be greatly appreciated, as well as general tips or advice. I can't wait for our trip!

Another question - should we take the train to Ottawa for one day while we are there?

The hotel has a very good reputation. The Vieux-Montréal is a must see, so your hotel is well located. You'll find plenty of restaurants in this quaint neighbourhood (although many touristic ones). Unfortunately, the Vieux-Montréal is not as lively in December as it is during warmer months.

There's many things to see, it all depends on your interests. For the most interesting sectors, let's say one day in the Vieux-Montréal, including Notre-Dame and one or two museums (I suggest Pointe-à-Callière and the Centre d'histoire). An other day for downtown, to do some shopping on Sainte-Catherine and a few museums (Beaux-Arts, McCord, for instance). A visit to the Mont-Royal (especially the Chalet) is a must for the wonderful view. A third day for the Plateau Mont-Royal and the Quartier Latin, to see the non-touristic Montréal and have a real feel of the city (St-Denis and Mont-Royal streets, Square Saint-Louis, ...). Great neighbourhood for restaurants. The Jardin Botanique is an absolute must see from Spring to Fall, but unfortunately, the great exterior gardens don't present any interest in December for obvious reasons.

If you want to do a little trip, I strongly suggest Québec (the city) instead of Ottawa, no offense to the federal capital. It is a real gem, worth at the very least two days. The real interest of Ottawa is its fantastic museums, not really the city itself.

Yacko had some very good advice. My main comment would be that Hotel XIXe Siècle is a very nice hotel, but the Old City is a bit away from the downtown and the plateau, and 4 nights there might be too much in winter, when walking is more difficult. You may wind up using taxis quite a bit. For the first time visitor, the old city is quite enchanting, but it's fairly small and can be seen in one day. The downtown and Plateau areas are very extensive, and full of interesting stores, galleries, restaurants (particularly the Plateau), and clubs.
My favorite museums are the Beaux Arts, and the Redpath (on the Mcgill campus).
I agree that for a one day trip, Quebec City would be more memorable - although if you are a museum person Ottawa is great.
Yes, bring lots of layers and good boots.

There's no doubt that Québec City is a marvellous place to visit, but for a day trip in early December, I'd recommend Ottawa.

Many of the delights of Québec are to found in walking the streets of the old city. At that time of year, it will likely be colder and snowier than Montréal, and unlike later in the winter during Carnaval, there won't be much in the way of winter attractions.

Ottawa, on the other hand, while still cold and snowy in December, has many indoor attractions. Others have mentioned the museums and certainly there is more than a day's worth in the Canadian Museum of Civilization, the National Gallery and the Canadian War Museum to mention only several of them. Québec does have some very good museums and galleries, but they don't match Ottawa's in my opinion.

A plus for Ottawa is that the train ride is 2 hours as opposed to 3 for Québec, so you would save 2 hours on a round trip.