rpenmanparker, I think the angle of the chain during disengagement/engagement will have a wear factor. SS and FG bikes have little or no deviation from a straight chain line.

I haven't kept records, but my impression is that replacement intervals are roughly two chains for one cassette and two cassettes for chain rings. So it does seem that the larger the toothed gear, the longer lived it is, again I can't prove that.

Brad