Understanding how to analyze a SFR vs. Duplex

[Fred Shandler]

I recently saw a SFR and a duplex for sale. Both had similar asking prices with similar gross rents. I was a little confused, however, when trying to analyze both deals, as the SFR seemed to be significantly better than the duplex. The following are basic numbers for the purpose of easy calculation:

Single Family
PRICE: $30,000
RENT: $650

NOI: $325
30 years at 6%: $179 P & I
CASH FLOW: $146

Duplex
PRICE: 30,000
RENT 1: $325:
RENT 2 $325

30 years at 6%: $179 P & I
CASH FLOW: $146 ($73 per door)

So, what does a scenario like this indicate? Could someone explain how/why the SFR would be a better deal, or am I approaching this scenario from the wrong perspective?

Thanks!
Fred