I've done both, plenty of times, but let me tell you about my favorite occasion.
About 10 years ago, I was traveling on business, and feeling quite horny, so I connected with a couple online. He was bi, and she wanted to be with two guys. I was in my late 30s, and they were in their early 40s. He was thin and tanned and squeaky clean. She was curvy and tanned and, well, a little bit drunk when they arrived at my hotel room.
We all got down to business in my big king-size bed. She lay back and wanted us to take turns going down on her. I went first, and he knelt at her face, hoping she'd suck his cock. She dabbled at it, but really wanted to just receive the attention. I spent a few minutes eating her pussy, and eventually he took over. After a few minutes, he wanted to fuck her but she wanted more oral, so he kept going.
While he was doing that, I paid attention to her tits, fingered her clit, sucked her toes, etc., and really did my part to make her feel good. The guy had been kneeling between her legs to eat her pussy, but eventually got tired and rolled onto his side as he kept up his oral attention. I noticed that my friend's cock was twitching and dripping precum while he was in that position.
I thought about moving down to suck him for a while, but I was busy sucking her nipples, and she was in a groove, so I didn't want to disturb it. (If there's anything I've learned about sex, it's that if a woman is enjoying the ride, don't stop the car.) Anyway, my friend's cock kept twitching away and she kept moving her hips rhythmically.
Finally, he needed relief, so he knelt between her legs and pointed his cock toward her pussy. "Eat me for two more minutes, and you can both fuck me," she said. He got on all fours and buried his face in her box, but I noticed he was jerking himself lightly, so I figured he was on the edge. I abandoned her and moved around to the foot of the bed, where there was an upholstered bench I could kneel on.
I gazed for a moment at his small ass, and thought how girlish it looked, before I spread his buns and began tonguing his hole. He started panting immediately and stopped eating his wife's pussy. He braced himself with both hands on the mattress, hung his head, and his breath got louder. I reached between his legs and lightly touched his balls. I felt them spasm in my hand as he suddenly grunted over and over. I made him cum hands-free, just by tongue-fucking his ass.
He rolled over, exposing his cock to me. I reached over to finger his wife as I leaned in to suck him clean. Then I buried my face in her pussy and brought her off fairly quickly. Apparently, seeing her husband cum from the attention of another guy turned her on and got her off. She let me fuck her — as a reward, I guess — and we were all happy. We ended up spending the night in my room, in my bed, and we had another, more conventional, round in the morning.