Yep 1080I is interlaced If im not incorrect saying so i think Its Vertical Pixels then Horizontal Pixels. So yes slower for sports and will give tearing affect. 720P is constant Hi-Def Pixels But Not as high resolution. 720P has been arguably the better contender.
Im not sure about canada but im pretty Sure American TV Fox network broadcast in full 1080P Progressive, which is far better but you will not see quiet as good performance increase from Normal tv to 720P Because of The Law of Diminishing Returns :P.
So If you can get 1080P do that, But otherwise 720P is the way to go.
Any Further questions about anything Next Gen Tv ask me.