Montreal - Old Town or Plateau district?
 

First time visitors to Montreal. I understand there are two unique districts in Montreal. Old Town and the Plateau district. What is the difference? For a 2 or 3 day stay is one better that the other for accommodations and restaurants, etc. I've heard the Auberge de la Fontaine is a nice hotel in the plateau area and the Marriott Chateau Champlain in old town. Any help in selecting between these two?

Both the Plateau and the Old Town are very desirable areas - it depends on what you want. The Plateau is a somewhat 'hip' more French speaking neighborhood of interesting restaurants (many BYOB), funky shops, outdoor cafes and French Bistros. Most of the side streets are quiet and residential with small parks, and lots of bike lanes.
Old Montreal is on the waterfront, with cobblestone streets, horse drawn carriage rides, very old architecture, and many good restaurants. The streets are often swarming with tourists. The down town (another good area to stay) is in between the Plateau and Old Montreal. We've been to Montreal dozens of times, and prefer the Plateau. You can easily get around, so staying in one area doesn't rule out enjoying the rest of the city.
As for your two lodging choices - they are vastly different. The Auberge is a small somewhat upscale b&b/hotel in the heart of the Plateau, across the street from a very pleasant park. Great location!
The Marriot Chateau is a high rise chain hotel with a pool, exercise room, etc. It is NOT really in the old city, but closer to the downtown in a somewhat business area. However there are many small boutique hotels in Old Montreal if that is where you want to stay.

There are far more than two unique districts in Montreal, but the two you mention are two where visitors commonly stay. Zootsi sums up the differences well between your two neighborhoods choices, which are both fine in my opinion. One thing to be aware of is that not many Montrealers actually live in Old Montreal (or Vieux Montreal, we don't call it Old Town), which means you're in a mostly tourist and Montrealers-out-on-the-town-or-working type of place. This is not to diminish the neighborhood though, as there is charm, some great restaurants and beautiful architecture.