It seems fairly clear if you consider a 50-100 year roof (ours is 150 years, and it's only had membrane changes in that time) against a 25 year roof.
I'm sure the steel shingle will last a fair time but if the PV elements need replacing four times a century, that's not a non-trivial cost.
When my PV panels die, it's just £400 a panel, four hex bolts and some quick connectors to replace it. It's no contest.
