Questions to consider are:
-How much can you rent the other side out for? If the delta between the SFR and duplex is a cool $500,000 you'll need to be renting that side out for *minimum* $4000 or so to make it vaguely worthwhile.
-Do you want to be landlords/share a building with tenants?
-What are the applicable laws like where you live? Very pro-tenant? Very pro-landlord? How hard would it be to evict a bad tenant? Because if you need to evict someone who lives on the other side of the wall from you, that's a whole lot bigger of a deal than someone who lives in a place miles away.
-Can you actually get a loan for a million bucks? Because the lender won't generally count theoretical rental income when determining whether you qualify for this loan.
Given the info you've provided so far, I'd probably either keep renting or do the $750k (ouch) fixer-upper.
-W