Well, I've tried both and my personal preference is Stormy Mondays - they sound SO much more like my own imagined idea of "the PAF sound". Mules are fab too, you can't go wrong with them... but when I got my Stormy set in January I suddenly found the sound I'd been looking for ever since I started my BKP trip.
However, two reasons I think you might need to put the Mules in the R8:
1. "I use this guitar in my 70s style Punk band" - for my money, out of the two, that's Mules not Stormies (actually, I'd go Riff Raffs). But if you get more gigs with the Rock covers band, then I'd personally lean towards Stormies.
2. "Whichever set I don't put in the R8 is probably going in my SG" - if had just a Les Paul and an SG, and a set of Mules and a set of Stormies, I'd put the Mules in the LP and the Stormies in the SG (well, obviously it depends on the specific guitars, but I'd start out trying it that way).