It is a combination of the the two. If I had to choose, thickness is #1, no, wait, length to reach deep inside of my pussy. I seem to have a g spot deep inside my pussy that only a long cock can touch.
But then again, there is nothing like be stretched to the limit by nice fat cock, by fat I mean I cant get my hand around it at the base.
Of course, I must have my clit rubbed to cum, that way I have vaginal orgasm as well as clitoral.