This is one of the classic struggles many couples experience... and there's actually a third option here as well: to remove the mouth completely before ejaculation begins.
It's well known fact that most men who enjoy BJs prefer that their partners swallow - assuming that they don't belong to more visual camp who enjoys pulling out to see themselves ejaculate on their partner's face or chest. It's also a well known fact that lots of women (not all!) dislike the taste and/or texture of semen.
The first important question to answer in this debate is why exactly do guys want their partners to swallow. This is something that many couples don't really think about and often if a guy is asked why, he might say something along the lines of "I dunno, just because" or "because I like it." Here are some more thought out answers:
1.)Biological hard wiring. Even though oral sex is not vaginal sex, the subconscious mind doesn't differentiate. Guys are programmed to want to "cum inside" because that's what makes babies.
2.)It feels better. Swallowing allows a guy to have an uninterrupted orgasm. He gets to progress to climax, ejaculate and fully enter into the resolution phase of his sexual response all while experiencing a consistent sensation. This is a much better experience than quickly switching over to a hand job just as a guy begins to have an orgasm.
If the person giving oral sex opts to spit, it's important to continue the same motions until the guy has completely finished his orgasm. As long as a guy has the psychological satisfaction of "cumming inside" and the physical satisfaction of not having his orgasm interrupted, it doesn't really matter what happens to the semen afterward.
But let's think about this logically for a moment. At the point that the giver of oral opts to allow the guy to ejaculate inside of his or her mouth, swallowing is really more efficient than spitting. For everyone who dislikes the taste and texture of cum - consider the two options:
1.)Swallow the semen as it's ejaculated, moving it directly from the penis down the throat. In this scenario, there is brief, minimal contact with the tongue and the rest of the mouth versus:
2.)Letting him ejaculate in your mouth and holding it all in until he's done - which means taking it all in and maximizing the contact it has with taste buds and time it spends in your mouth.
If taste and texture is what you dislike about cum, swallowing makes a ton more sense. If swallowing cum causes you to gag, or cum upsets your stomach, then you may have a case for spitting.
The real debate, however, is whether or not to allow a man to ejaculate inside the mouth in the first place versus quickly removing the mouth before ejaculation begins. This fundamentally becomes a question of giving in a relationship. Men are frequently called to consciously be aware of their partner's needs. Since most men orgasm relatively easily, we aren't always conscious of their needs and desires. Maybe getting semen in your mouth isn't the most pleasant experience to you, but stop and ask yourself what sorts of sexual accommodations your partner has made for your pleasure.
The best relationships are those that exist in cycle of giving between partners. Instead of getting hung up on and arguing about what each of you refuse to do in bed (and around the house), sometimes it's actually easier to just do them and then see how happy it makes your partner. Then something that was unpleasant and the source of contention in the past, becomes something that you actually want to do for your partner.