[Q] Compare duplexes. 2 units beside each other or on top of each

[Harry Smith]

Would you rather buy a duplex with 2 units beside each other or one with 2 units on top of each other? (assuming you live in one and rent the other) (located in state of California, if that matters). What are pros and cons?

From standpoint of maintenance, appreciation, tax, comfort, etc.

Thank you very much!