Eventually everthing will need to conform to the 2006 emission standards, but both 4-stroke and 2-stroke with direct injection can meets these, but you will still be able to use the older motors. I have heard some lakes require 2006 compatible engines in certain parts of the country, don't know if this is an item to consider.
I would also check the pricing on a 4 stroke in Canada. I have purchased two (one at the begining of last year) 4 stroke honda 9.9s for less than you are looking at the Merc and Nissan.