It would be cheaper for you to just do unlimited with sprint with international add on for about 5$ a month (like 7c a min or so to canada) or pay an extra 15 to use your anytime minutes.
Att is about 4$ for the canda monthly add on and about 19c a min for wirelss/landline. I can tell you that Att has overall better service than sprint (stateside) but depending where you are sprint has the advantage and will be cheaper in the long run due to unlimited. Im a GM at ATT and have both sprint(personal) att(business)
As for Verizon? Verizion has the best service in the states…no doubt. Its not even close but, they also have the shittiest plans and are pretty expensive compared to Sprint/Att. From what i remember i beleive verizon has a nationwide plan that shares with your anytime minute plan. Just know that you will be spending more money with verizon bc your data packet is going to be seperate service they do the whole “share everything” bullshit.
So again Id say go with either sprint or att. In the end it really is just gonna boil don to how much data you use…if you use alot? stay far away from verizon…