In general, having lived in a 2-story home for many years, I personally prefer single story floorplans for the following reasons:
1) Over time, I did not like having to traverse the stairs multiple times every day and cleaning them was a pain. Also, moving furniture up and down stairs can be a challenging.
2) Stairs are a physical barrier and safety issue for some. Given the large number of "Boomers", this will reduce demand from this market segment when it comes time to sell.
3) From a seismic perspective, wood frame single story homes are more resilient to ground movement.
4) In general, two-story homes are more expensive to construct and/or modify than single story homes.
5) Under the right circumstances two-story homes can be more difficult to evacuate during a fire.
6) Like heights? Try painting the vertical walls of a two-story home without getting vertigo! Having to call a roofer to repair all the newly broken roof tiles you/or the painters created is no fun either.
7) I have walked a few homes in my day and I find more floor issues (squeaks, slight elevation changes, bumps, etc) in two-story homes.
We've owned a single family ranch in North San Diego County for 32 years and I can't tell you how many RE professionals and not actually knock on my door and ask me if I'm interested in selling. At 67 and having had a double knee replacement ,not to mention my 12 Grandchildres, I'm not about to part with this beautiful piece of property anytime soon.
I am confused by your reference to a duplex ---- typically, a duplex means two separate living units for two separate households. And each unit will have a separate address (and each will have its own functional kitchen and full bath to be considered a separate unit)
But what you're describing seems to mean that one property has a bedroom and a bath on the first floor, with the rest of the bedrooms and baths upstairs. That is not a duplex -- it merely means a 2-story home.
If all things are equal --- location, amenities, size of lot --- the preference may be for single story, particularly as people age or have health issues and cannot manage stairs