In Ontario once you surpass 9920lbs (4500kgs) Gross weight, you need a CVOR and commercial registration. Staying below that you can save a lot by not tossing the government more money. The higher payload could appeal more to contractors but if I considered a 2500 gas for a work truck I would stay with the 5.7L. The 6.4 here in Canada commands a $2K premium and larger displacement despite cylinder deactivation will knowingly eat more fuel than the 5.7L under similar circumstances. The 6.4 has better built internals so it would be preferred when towing heavy on a regular basis and a diesel was out of the budget but for me (property maintenance) the 5.7L would be plenty with its 2300lb payload.
I just looked, and the Canada spec for a 2500 6.4 has a 10,000lb gross, where the 5.7 has a 9,000lb gross which accounts for the 1K payload difference but with the 6.4L when using for business purposes you must register (at least here) for your commercial vehicle operator registration (CVOR). The 6.7 diesel also grosses at 10,000lbs so the payload is similar to the 5.7L gas.