My preferences changed a after the first few times we enjoyed mfm. We didn't yet know about all the "cuckold" fetish stuff, but we both wanted me to be more submissive and simply help her reach orgasm. Our style of playing changed over the years but we both love watching each other in orgasm with others. When were first married and enjoying mfm with one of her coworkers, it was mostly missionary with me laying beside her and sometimes I'd move to the foot of the bed when I sensed his ejaculation was imminent. I wanted the full-on crotch view, which left me in the perfect position to enter her or eat her when he finished and rolled off.
My wife enjoys anal, mostly in the face down or prone position and I've come to enjoy that position as much as missionary. I like to kneel beside the bed at the top of the bed while she's being taken this way so we can talk about what she's enjoying. From this vantage point, I can watch her body flow into orgasm and I can see the sexiest thing a husband will ever see, - his body as he ejaculates deep in my wife's anus. They are both silent as this takes place, the only sounds are breathing sounds. Then his breathing stops and I know my wife is being inseminated. It is at this point that I achieve my firmest erections.