Wendy--

there are two answers -- one very quantitative and based on rental vs
usage cost. Then there's the qualitative answer which will usually
take priority. Owning the aircraft means it's always available (unless
it's in the shop), you know exactly who's been flying it and how,
and it's always fun to watch the reaction on non-pilots faces (even
funnier on men!) when you admit to owning an airplane.

And you never have to worry about getting home from a trip because
the airplane is reserved by someone else.