If i want to put more miles on it, and/or used more regularly then auto. No question, A manual will be more entertaining to drive, but using it day to day when you have other nice auto cars is less compelling for a quick run to town/ run errands with.
Somewhere approaching 40, my manual trans cars began sitting a lot. Disclaimer, I do have a 6 speed CTS-v project on the back burner in the corner of my shop, and i have no interest in letting go of, but I see it as take if for a quick 45 min rip on the back roads once a week or so, with an occasional solo/ travel lightly quick roadtrip type use, and if i gotta run errands on saturday afternoon, i'll probably take something else..