Depends what sort of music you mainly play.
I have both an 500L in one guitar and a 500XL in another. I would say out of the 2, the 500L is a good all rounder for all types of music. The 500XL is more higher gain & more trebly, and I find I have to adjust the treble knob down by half on the guitar (or amp). If you're playing
thrash metal I guess the 500XL is better, you get more squeal with harmonics, and can really hear the pick attack when playing solos.
My personal preference is the L500, you get a nice chugga chugga sound, and it's more versatile. I plain anything from Hendix,AC/DC right up to Metallica/Pantera with it.
If still undecided, have a look at other peoples reviews here
http://reviews.harmony-central.com/r.../Bill+Lawrence