Believe me, I agonized over the decision. But in the end, I couldn't get past the fact that the ticket offered by the contest was non-transferable. If I came down with a heart condition, or diabetes, or some other condition that would prevent me from passing the medical screening, nobody would be able to take my place.
However, after speaking with Space Adventures, I learned that their standard Terms & Conditions have no such limitation, and contain provisions for transferring the ticket in case of illness, death, or whatever. So if something happens to me, I can give the flight to my son, or brother, or somebody else that I know would appreciate the experience.
To put it in terms everybody can understand, I didn't like being the single point of failure
I've already talked to Space Adventures, and committed to becoming a customer as soon as I have the funds available.