well, i haven't tried the js100, but it has a cr@ppy trem, made in korea, while the 550 is made in japan and has an awesome trem.
http://www.jemsite.com/ibanez/spec_js.htmscroll down to the js100 bit. Note it says "cheap guitars- avoid them!".
get an rg550.
:D
EDIT: although you already have a js100. I think the neck profile is different. the js's have a vintage type neck, the rg's is wider, bigger frets, more for shred. So, although it's technically a "better" guitar (better construction, parts and wood), you might not like its "feel".