Cowgirl! After forty, life on the bottom takes on a definite appeal: gravity is my friend, and everything I like is right in front of me! Contrariwise: there is nothing like the missionary for initiating a relationship, where the two lovers can look into each other's eyes from very close range at a critical moment! So, I guess my answer would be: all things in their time....
Oral sex: before or after the main event (this definitely goes to level of commitment)?