Rent est. over-under vs side-by-side duplex

[Nick Detloff]

I’m new to estimating rent. I’m looking at a couple of different duplexes. 

One is a side by side each unit having its own single car garage. The other is an over-under with no garages or covered parking. 

Looking to gain an understanding on (1) whether or not over/under units typically sell at less than side/side units and by what approximate percent less, and (2) is it smart to estimate less rent for units without a garage/covered parking? Approximately how much less?