Re: 6 or 7???
I have tried low-tuned 6 stringers, baritone 6 stringers, and 7 stringers. Here's a few things I noticed about them:
-low tuned 6 stringers need to have HUGE gauge strings to keep them in tune. If you're used to 8's or 9's, the commonly used 12's are going to feel like telephone cables to your fingers!
-baritone 6 stringers solve this by adding scale length (27-28" is not uncommon). This enables you to use lighter gauge strings (say 10-11's) that will still stay in tune.
-I find that baritone guitars are uncomfortable to play, due to the shifting back (towards the rear strap button) of the pickups, bridge and tailpiece. You're really going to have to s t r e t c h those arms apart to play low on the neck. The ESP Lynch flame baritone suffered greatly from this. It just felt weird and uncomfortable to play.
-7 stringers KEEP the high 'E' string, whereas low-tuned and baritone 6 stringers only have a 'B' as a top string. This cuts your soloing scale way back (5 frets on the top end). It's like having the equivalent of say a 17 or 19 fret neck on a normally tuned guitar. This really isn't a factor if you're planning to just grind out some Korn type low end chunk. If you had planned on some Petrucci-esque shredding, you're out of luck!
-7 stringers have really w i d e nut widths. I've seen a few that come damn close to 2" at the nut. Unless you have big hands or longish fingers (monkey-like!), you're going to have a hard time with 7 strings.
-I have two 7 stringers, and like you, I'm considering adding a Viper 407 to my already too huge guitar collection! Best advice- drive down to your local guitar-mart and try a few of them out for yourself! Good luck, bro.