i can't say for canada, but as far as insurance goes here, most companies i have seen will require a commercial policy for any vehicle that has a non-standard body. i was told by multiple companies that even if you have an old 1/4 ton truck with a rusted body, if you replace that with a flatbed (metal or wood) it technically requires a commercial policy. some will even require that if you attach a snowplow. they never tell you this - you only learn by asking. a friend's mother who is in the insurance business mentioned the commercial policy to me when i was buying my 1 ton dump. i used a different company but when i asked my company, they told me the same thing. it's all a matter of whether the company will refuse a claim based on this. i don't know if they will, but for all the money we pay, i'd hate to find out. better safe than sorry.
another thing i found with the commercial policy - my company automatically covers any trailer i tow (liability) along with my 3/4 ton (non-commercial) for no charge. my 1 ton requires a separate coverage for each trailer i tow. it's not very much money, but it's kind of a hassle.
for fuel mileage, i can't imagine there's a lot of difference between automatics and standards in newer vehicles. even automatics with lock up converters and 5+ speeds are probably pretty close to standards.