I like both. I've never been able to give multiples any other way than by oral, and it's cool to feel her pussy clench and release as she cums while I'm going down on her. Almost like those pussy lips are kissing me back. And getting my beard and moustache soaked in her juices is a big turn on too.
The most frequent way to get her gushing though, by far, is by having her sit on my cock. I find that once she starts cumming like that, and all her warm juices start flowing over my balls, and running down the cheeks of my ass and thighs, I'm not going to last another minute or two.
I see no reason to have to choose........do both! Sounds like a win-win deal to me.