We prefer duplexes to SFR's because you're not entirely dependent on one tenant's rent. We bought one in 2011, renovated one side in about 2 weeks, rented it, then renovated the other side and rented that one about five weeks later. That puts the tenants on different schedules (their leases don't end at the same time) and reduces the risk that we'll ever have a completely empty house with no one paying us any rent.
As for desirability, as the second poster said, these are two different types of property that are desirable to different kinds of people. You just have to use common sense here. Like, who lives in what neighborhoods? If you're looking at a neighborhood that seems attractive to young professionals or grad students, then look for duplexes because such tenants just about never want more than 3BR, usually they want 2, and if you get a duplex with 4BR (2 on each side) you can rent it for more than a 4BR SFR.