first post! YEHH!
NJ is neck thru, acrylic is a bolt on. NJ will sustain way better just because of the contruction.
NJ is made out of nato (cr@p) which kinda sounds like mahogany, so decent tonality. Tuners and pickups suck, but can be easily upgraded. All of that aside, the NJ's are probably the best production model BC Rich's you can get, theyre well built, sound good, and can stand up to some serious punishment.
For the acrylic though, it will both have a tight thundering low end, and screaming highs, a really cool tone, but its not for everybody. The only thing i'd worry about is the tuners -- which MUST be upgraded if you plan on using strings above .10 gauge or it will never stay in tune-- the pickups, because bcr bdsm pickups are pure shitee, and the neck.
On the low end models, BCR very rarely makes a decent bolt on neck, and even more rarely does a good fretjob. But hey, if you go out and play one and the neck and frets feel like sex under your fingers, by all means get it and dont worry about a neck mod.
I'd say go out and play both of them and see what you like.
EDIT: added
Ps, dont get the Mockingbird special X, its the lowest end version, bolt on. You can definately do way better.
