I own both versions of the GB, two PM2s (s30v and "blurple" s110v) and have handled a few Manix models. I liked the Manix XL a lot, I do have very large hands though. If I had to recommend one...
I think that the Paramilitary 2 is the best EDC knife on the market. The entire design is very refined, besides cosmetics/steel choices I do not see how you could refine it more, for what it is. The tip is very "splinter picker" style and it's everything that I like about the full size Military in a smaller package. Sometimes it fits into the smaller places and just handles a bit better when I need mobility. The standard s30v satin blade has treated me very well. I like the s110v "blurple" but my example is flawed, I have been putting off sending it back to Spyderco. It needs some work on the lock bar as there is a huge burr that makes it bind and stick like crazy. That is beside the point though...
If you are doing heavy duty cutting the GB1 is the best folding knife I've ever handled for that job. Ripping through zip-ties, cutting wire... I cut heavy plastic often and it SLAYS at that. I suppose it depends on what your use for the knife is. I'm not sure what your daily use for a blade would be but the GB1 is a heavy duty cutter. As for the GB2, it is a nice design but I much prefer the original. The front choil on GB1 just fits my hand better and I enjoy the blade shape more.
My recommendation, finally? I'd grab the Gayle Bradley 1 as it is discontinued. After that, I would immediately grab a PM2 as it is the best EC knife on the market hands down. Between those two you have essentially the best that Spyderco has ever produced.
To speak on the dust and wear on the knife/locking mechanism issue, I would imagine a liner lock or comp lock would be the better option. Nod would go to the comp lock. Grit in a backlock is an SOB and I base that on experience. I'm not so sure it matter though, do not lube heavily with oil as not to attract dust or dirt and you should be fine.