Well, since not many people have replied, I'll give it a shot. I don't have much experience with the fairleads, but my friend has one and I have one (but I'm still in the process of making my bumper). He used steel cable and the plastic fairlead for 1 run. He got stuck in a mud hole on fordyce and had to winch the front of his truck almost straight up to get out. Doing that did wear a groove into the fairlead, but it's a pretty extreme case. The groove isn't too big so it's not like it ruined it or anything.
Another thing to keep in mind is that the fairlead will tend to deflect up or down if you're doing an up or down pull so it's a good idea to weld a tab above and below the fairlead to prevent that. Personally, I'd recommend synthetic line with the synthetic fairlead.