i want to buy a surefire for my 870 but not sure which foregrip is better the 918 or the 618 with the system disable switch...any info on which one is more worth the money

I guess it kind of depends on your needs.  

I bought the 6v without the system disable switch.  I think that for indoor use, which is the majority for me, the 9v would be too bright and the reflection of light off of walls, etc. would be bothersome.  I also don't see a need for the system disable switch for me.  I just can't imagine when I would use it.

Bottom line, for me, the least expensive version also completely met my needs.  Your needs may be different.