OP has not provided enough info for a reasonable analysis. You need more than JUST these numbers...like it or not, RE involves those fuzzy things- emotions, feelings, etc. Realistically, a SFR is not generally a great deal different than a duplex when you look at the big picture. Specifically, how does the lot SF, finished SF, floor plan, style, location, and orientation compare?
If the subject duplex has roughly the same finished SF as the SFR, then the rooms in the duplex will be cramped and not as useable, and thereby harder to rent. OTOH, if the duplex has been upgraded, but the SFR is at the end of its functional life, the SFR will be harder to rent. With the subject properties mentioned, clearly there is a significant inequity that you need to identify.
Can you force appreciation with minimum effort on one or the other?
Which one is the better deal? Which one provides the most total current value, and which one has the potential for most improvement? What is the 10 year REAL projected maintenance expense for each (I'm not talking 50% rule here-- I mean have you surveyed the physical elements and established a realistic repair/cap improvement schedule for each property?)?
You need to compare apples to apples, or at least KNOW that you are comparing apples with grapefruits. You should know your locale well enough to have a baseline " per unit" acceptable cost range for a " typical" unit of the type and condition you seek; and, you should have a ballpark idea of each typical major repair item you typically run into. If your ballpark figures don't get you into a range that will work financially, there is no need to sweat over figuring it down to the penny. There should be a clear profit + fudge factor minimum that you use for go/no go decision, but you need to factor in all of the tangible, and intangible variables.
