Interesting thread & postings. I have not had the BM bone collector, so I can't really comment about it, although I have had many knives in D2 including several BMs (Both folders & FBs). I've also had the aluminum handled version of the BM in question along with several Griptilians & a couple Wilkens/Grips. I currently own a Gayle Bradley & have had 2 different Stretch 2s.
I think the BM & S2 sit wider in the pocket, although they might be lighter. ZDP-189 is an excellent steel & will hold its edge for quite some time. It can chip, but should be just fine for the needs of a folder.
The GB is incredible. F&F is very good & though it's fairly heavy, it doesn't feel that heavy (About on par with my lg. Hogue). M4 might not quite have the edge holding of ZDP, but it's much tougher & the edge can be thinned out & still be tough (That's why it seems to dominate in chopping contests nowadays).
Of the 3 steels, M4 is the least rust/stain resistant, followed by D2, then ZDP-189, which is a stainless steel, though it'll stain easier than many SS.
Comments about the GB's liner lock is interesting, but I think Spyderco did an excellent job with this one. It's fairly thick, with 1/16" thick lock & liner. There's no indentation & the lock doesn't stick out, so I don't see this one being easy to accidentally unlocking.
People question the liner-lock's durability, but I'm seen many back-locks (Including Spyderco's) developing blade-play (Usually up/down), so I'm not sure the GB's lock is really a detriment compared to the others.
The axis-lock on the BM is excellent, I'll admit that & have had no issues with the ones I've had.
I think all 3 knives are excellent, but prefer knives that aren't so wide in the pocket. Something about the Stretch 2 just doesn't do it for me, not sure why. Personally, I like the GB best, but admittedly have not had the BM Bone collection model.
I do agree that you should just wait till it comes in & then see if it's a keeper or not.