Steven,
Odd answer but I would say both.
My first house I lived in was a property with two dwellings on it, which were not connected (a.k.a an in-law unit). I was able to live in one while renting the other. If you're trying to maximize your monthly cash intake (after mortgage and other expense are paid out) then often times having multiple units on a single property yields more cash.
Their are drawbacks of course. Usually duplexes are located near or around other duplexes and multi-unit dwellings. So appreciation of the asset might suffer as a result of the condition of the other homes in the vicinity. Personally, I prefer to find properties with 2 units embedded in neighborhoods with single family homes so that you have a stability of the surrounding homes helping to maintain (and hopefully increase) your equity.
There are some great resources on the site if you haven't already explored them.
Hopefully my 2-cent helps.
Regards,
Nadav