Hi there :D
As Tom mentioned, I have tried both Alnico II SMs and Alnico IV Mules in a Les Paul Standard.
Whilst both are good, I definately recommend the Mules due to:
1) A slight mid range 'hump' which personifies that creamy L.P. sound.
2) A bit more gain (both in the windings and the Alnico IV magnet) which also seems to to 'punch' sweetly in a characteristic LP way.
Both types of pickup are sonically very well balanced, and sweet sounding in the higher registers. The S.Ms are however more transparent sounding and totally balanced, giving an almost acoustic quality. A set of S.Ms in a Les Paul therefore seemed to make it sound more S.G. like.
I was told by Tim in an e:mail that he uses the Stormy Mondays as a basis for his 'Peter Green' specials, but I can confirm that when I took the Alnico II Stormy Mondays out of the Les Paul (for my S.G.) and fitted instead the Alnico IV Mules, it immediately sounded like a 'proper' Les Paul - without the annoying mud or harsh top end of the Gibson stock 490R / 498T pickps.
Also if you wire one pickup out of phase with the other via a push/pull pot on one of the tone controls to do the 'Peter Green / Snowy White sounds in the middle toggle position' trick, it seems to work better with Mules.
For me it is generally Mules, due to the modest midrange boost = creamyness.
Finally, (phew) I gather that the 'Mules' are closest in build and sound to a late 50s pickup,, whereas the Stormy Mondays are closest to an early sixties (i.e. Gibson S.G. era ) type.
Regards,
FF.