If I'm being masturbated at the time, then it doesn't matter so long as it's stimulating me somehow pleasurable. I'll cum one way or another sometime.
I prefer to ejaculate deep into the vagina, maxing out on that final thrust, which can be frustrating if I'm with a woman who prefers that I should cum in her mouth. I will always do what my partner wants, but once you reach that point of no return it's not always that easy to withdraw to make such a rapid change of orifice.
Pussy...I like the feeling of his thrusting when he loses all he has. However, if I can't have it there, my second choice is my mouth. I don't have a problem with swallowing.