As a bottom, I prefer missionary most of the time. As a top, I prefer (or rather, bottoms prefer) doggy due to simple anatomy. At the end of the day I still find myself trying a mix of all sorts of positions and at different angles to make sure both parties are satisfied. Also depends on what the mood is like. Is it a quickie or a longer, more intimate session? Frankly, penis shape, length and girth all play a factor in hitting the prostate, but eye/body contact and other little things play a part in establishing intimacy. How I choose to balance everything out depends on my and my sexual partner's preferences.