I've never lived in a house with a basement, so I wont comment on the pros or cons of one, but I will comment on stairs and getting older. How old are you and your wife? Is this going to be the house you grow old in? Is there any history of knee problems? hip or feet issues? Any mobility issues at all, or is there a chance of having problems climbing stairs?
I made the mistake of putting two bedrooms and a bathroom upstairs when I built my house. I'm 52, getting overweight, but still work every day remodeling homes and consider it to be physical work. I hate going up stairs and probably only go up my stairs in my own house half a dozen times a year, and that's to help my wife with something she has up there. We are adding a second bathroom to the downstairs of our house this year because she doesn't want to share one with me, and she doesn't want to go upstairs to use that one when I'm in the downstairs one. Avoid stairs, do not build a house with stairs.
The best houses are all on one level with every door being 36 inches and the hallways being even wider. Floors are all super easy to clean, and the walls are so well insulated that you can heat the house with a candle. Smaller is better. Less to clean, and less to pay to heat and cool. Besides the bedroom, kitchen and bathrooms, we all spend most of our time in one room. The rest of a house is either storage, or wasted space. Who really wants guests coming over, and how long do they stay?
Just my thoughts.