I think Québec City wins out for the Valentine's Day weekend, but partly because I am longing to revisit that lovely little city myself! Both cities can be charming and romantic, especially if you can book a room in one of the historic hotels like the Chateau Frontenac or Chateau Laurier. Beautiful hotels (but pricey)!

Carnaval de Québec is on in Québec until February 16 so there will be fun and special events around town. http://www.carnaval.qc.ca
has information on the events but I don't know if they have much English on their site.

Not to be outdone, though, Ottawa has Winterlude! Same kind of thing, really.
Skating on the Rideau Canal in Ottawa is kind of unique! I enjoyed a lot of live theatre when I was there last, as well as art galleries and museums.

I think Québec is a bit more colorful and has a bit more flavour, but others may disagree.