Well I use to own 13 JEMS and 3 UV's and now own 7 JS
guitars. Both models shred without a doubt. The JS
has more tone options though because of the coil split and hi-pass filter (which by the way is amazing when you turn down the volume it keeps it clean and sparkly on tube amps). Now these are things that can be moded on a JEM so I would not base my opinion soley on them. Majority of JEM guitars have PAF's in them so you SHOULD feel at hom with the tone of the neck pickup in the JS
since it is a PAF unless of course you are looking at a DBK, VWH, JEM10, JEM90, or BSB.
I played JEMs for a long time but when I got my first JS
guitar it was just incredible. The body just "conforms" to you and with the neck being a little "thicker" the harmonics are a bit more rich in tone (luthier fact: the thicker the neck, the better the harmonics) Thats why Eddie gets those insane harmonics
. Plus if you get the multi radius fret board you get the lower action (yes my fellow JEM fanatics it can get even lower!!) lol
I always thought Rich was nuts but turns out he was right!! (At least for me). When it all comes down to it though, just sit down with your amp gear
and a JEM and JS
and play em' both. Make sure you listen to the tone because things like action and intonation can be adjusted if its not "just right".
Hope this little ramble helped in some way!! Happy hunting,