I've stayed at neither but they are where I will stay on the next trip to Montreal; hotels are equivalent though quite different (starkly so!). St. James is larger, more traditional, in a beautiful old building (really outstanding architecture) and is an LHW which probably has the highest standards of any chain. Gault is more modern and has won many design awards. Montreal hotels have improved by an order of magnitude with the opening of these 2 hotels. Doubt you will find anyone who has stayed at both -- they are too new and if as good as they seem to be there would be no reason to stay elsewhere. I'll pick based on availability and value; in my judgement these hotels are equivalent, and far better than anything else in Montreal.

You might be interested from the following comparison of these 2 hotels that I saved when I was researching where to stay in Montreal:

"The most sumptuous is the Hotel Le St-James, which was fashioned out of a 130-year-old merchant-bank building in the Old Montreal district. Travelers looking for suites with antique furniture and plasma-screen-TV-equipped, marble-encased bathrooms can alight here for $300 to $3,750 a night. A bit pricey, non? "Our guests don't ask the price, they ask for the square footage," sniffs the hotel's directeur-general Guy Luzy. If price does matter, try the Hotel Gault (514-904-1616), a minimalist gem where classic 20th century fixtures and furniture are set against bare white, oak or hot-rolled-steel walls and concrete floors. Tucked away on a quiet Old Montreal street, the Gault's design aesthetic is so gaunt that a visitor can barely make out the insignia by the front door. Rooms are $170 to $435."