I didn't try the Samsonite Savor 18" because it's not available in Canada. The Samsonite website has this to say: Overall Dimensions: 18.0" x 11.0" x 9.5" which according to customer support includes wheels and handles etc so it has a high chance of being OK. It's also 160 CAD by the time it gets to Canada.
Instead, I've gotten
http://www.amazon.ca/American-Touris.../dp/B00CRTUFSE this 30 CAD bag. Cheap as dirt and the measurements are right for United and if you pack it so that the bag cross cut forms a triangle instead of a rectangle (pretty close to a square, in fact) then it quite probably would fit even the Air Canada stringy thin personal item sizer. You can do this because while the bottom (obviously) and the long sides have some structure the shorter sides have absolutely none. The Amazon product page has exact measurements in the question section. I am 6'3" and could roll it just OK -- it has two section extending surprisingly long handle -- but still I think I'll get an extender for it for comfort.