I did a 4 day Carnival Victory cruise to those ports a few years back. On the way up it was cold and stormy, not very very bad but cloudy skies and rain, rain, rain. When I got to Canada it was cool, with sunshine in Halifax and then in New Brunswick it was rainy and cold. It was in July too so just be prepared to deal with the weather because it is not pleasant. Also those cruises are very short, so you may not meet as many people and the trip will be over just as you get "settled" on the ship.
Personally I would take a 7 day to the Caribbean or at the very least the Bahamas/Florida.
Past Cruises: 10
Aug 2000 7 day Celebrity Horizon, Bermuda
Jul 2001 4 day Carnival Victory, Canada
Jul 2002 7 day NCL Sea, Bahamas
Jan 2004 11 day NCL Dawn, Caribbean
Apr 2005 7 day NCL Dawn, Bahamas/Florida
Apr 2006 9 day NCL Dawn, Caribbean
Apr 2007 12 day NCL Dawn, Caribbean
Jul 2007 8 day NCL Spirit, Bermuda/Caribbean
Mar 2008 7 day NCL Gem, Bahamas/Florida
Apr 2009 13 day RCL Explorer, Caribbean
7 day New York to Bermuda
July 18, 2010