Get it done in Canada.
A US seaplane rating WILL NOT be validated. As the others have said there is a solo requirement here in Canada. I have had students come with a US endorsement and then need to do the solo requirement.
You then spend more money getting checked out by a Canadian instructor before he will sign you off. No one is going to just kick you loose for the 5 take-off and landings if they have never flown with you. As an instructor I need to know that you have the skills required to receive the rating. Unless I can see those skills it would not be logical for me to sign you off and send you off in the companies plane.
So yes the states can be cheaper in the short run but once you come back it will cost you even more in the long run.