I prefer swallowing. Personally I find it more intimate and bigger turn on. Most guys I'm with though prefer giving a facial, which I'm not opposed to. I think porn has made facials the more popular thing to do because seeing a guy cum is a turn on for people. I do count myself in that group. Watching a guy jerk off to climax, landing a couple loads on his chest and stomach then watching the last bit just ooze out, covering his hands, cock and balls drives me crazy every time. BUT when I'm getting the facial I always keep my eyes closed, so i miss most of the show.
That being said, on the few occasions I've found myself on my knees sucking off two guys at once I'll always go for the facial. It can be tricky to get two guys to cum at the same time but i've found, almost with out fail, as soon as one guy actually SEES his buddy blow his load on my face it'll send him over the edge too.