If you tell, there's a good chance that he/she will break up with you. However, there's also the chance that he/she will be forgiving (which, I kind of doubt, because if you do it once, you might do it again).
If you don't tell, there's a chance that he/she will find out anyways through other sources, and that'll be worse than if you'd told him/her yourself. There's also the chance that he/she will never find out, but then you'd probably be guilty for the rest of your life.
If I cheated on my partner (which I highly doubt; I must've been high on crack or something), I'd tell him ASAP because I can't cope with a guilty conscience. It also works both ways because I'd rather find out from my partner than from an outsider. Of course, it won't be easy to speak up. I don't think it ever is, unless you're an "apathetic bastard" (I mean no offence to Aceman67, but it seems the perfect phrase to use).