(I'm assuming here that you're not talking about a situation like an open relationship where in certain circumstances other partners of your partners are explicitly allowed. Given that...)
To me they're identical in that they're both deception. Lying is, obviously, lying. Cheating is breaking an agreement that the two of you made. It may be a de-facto agreement or more explicit like in a marriage, but it's still an agreement that you both (presumably) entered into in good faith.
Lying is deception, and cheating is deception plus sex. Since for me sex itself isn't a hot-button topic, I see them as being pretty much the same thing. Neither is better or worse than the other. If it's an argument like, "You lied to me!" "Yeah, but you cheated on me!" where the cheated-on partner is trying to establish the moral high ground, then I'd personally see both parties as having shown themselves to be equally untrustworthy. Don't know if that's what you're talking about though.
I'm guessing most people would probably consider cheating to be a stronger deception than simply lying, but I don't see it that way. They feel the same to me. Truth is very important to me; sex is no big deal.