Older men and mature men are running pretty much neck and neck in Wordtracker for numbers of searches. I've used both and get equally good results.

Daddy is going to pull in yet a different crowd, but remember that in the gay community "daddy" often refers to a man who is masculine and in control, at least it does in my books. It's often used when referring to a daddy/boy relationship.

I don't hear the word "dad" used much in our community. Nor do I hear grandpa.

Cheers,
Michael