Hi Rich,
having tried both Stormy Mondays and Mules in my Les Paul Standard, I would say they are both excellent - but recommend you go with the Mules.
The SMs are totally balanced across the frquency range, whereas the Mules are equally clear, but with a slight mid range 'hump' that sounds perfect in a Les Paul.
As for the difference you can expect, I have experienced clearer bass, sweeter treble and more 'open' all round sonic delivery over Gibson 490R and 498T pickups. As someone else commented;
"it's like taking a blanket off of your amp"
I also read somewhere that the Burstbuckers tend to have a sharper, shriller treble - whereas the top end on SMs and Mules is very sweet and rounded whilst remaining detailed and singing like a bird.
I play mostly clean, so notice these details more. They handle gain nicely though and even the soundclips in the 'Players Section' do not do them full justice - as BKPs are very three dimensional sounding. You have to hear them in the flesh to fully appreciate the difference.
Finally, I gather the Mules are built like a late 1950s PAF and the SMs are built like an early 60s Patent number pickup.
I also am a Fender amp player, in my case a 'The Twin' - so have heard them on kind of "common ground". Just a shame you don't live in Yorkshire, you could have had a test drive
The reviews on Harmony Central are (in this instance) a good guide.
Regards,
Derek. :D