I'll help as long as you keep your 710-I love them so much. One of my faves.
The 030X knives are extremely overbuilt. Thick blade, tough steel, titanium framelock. They come in at a half-pound(!) I believe 8.0oz. They are great knives but the biggest complaint with these is the weight. But, the big plus of the 030X knives is they are assisted opening. This knife was designed with Strider and Ken Onion. It is a very solid, very cool knife. This knife has a re-curved blade which may affect one's ability to sharpen it. This knife retails around $270, but sometimes can be had for $200-$240.
The 0200 is also a huge 3 and 7/8ths" folder with big, thick steel, but this time in the form of a linerlock. Like the 030X knives, the 0200 is still a pocket hog, and heavy, but weighs in at 7.7oz vs. 8.0 for the 030X. Not a big difference in weight, but it is lighter FYI. This knife retails around $160, up from $120 as recently as a couple of months ago. This knife has a re-curved blade which may affect one's ability to sharpen it.
The 056X knives IMO are just improved 0200's and 055X's. Vastly, almost incomprehensibly improvements. The 056X has a ball bearing pivot, quite a bit better blade steel (Elmax vs 154CM), and weighs quite a bit less. This knife is the current flagship Zero Tolerance. Plus, it weighs only 5.8oz, which compared to the others is much, much lighter IMHO. This knife is also the only of the 4 with a deep-carry pocket clip, and it's a very secure, properly made one at that. This knife retails about $270 depending on availability. This knife will have additional scales from Hinderer shortly according to the word on the street. Peter R. from the forums here makes scales for this.
Also added to the mix: 055X (0551 was a limited run version with an Elmax powdered steel blade, and a different G-10 scale-Preorder only, no longer available). The 0550 is the predecessor to the 056X series. It has a CPMS35VN blade which is basically S30V with some Niobium added. This is also a powder steel like standard S30V, but a bit tougher, and reported to be even finer grained (Chris Reeve of Chris Reeve Knives fame helped design this steel-pretty cool). This knife has a very smooth action, but no flipper. It opens via thumb studs, and also sports a titanium frame lock like the 030X and 056X knives. This is the smallest knife here with a 3.5" blade. Even though it's a bit smaller than the 056X, it weighs the same 5.8oz. This knife is 8.125" in overall length, about 0.7" shorter than the 056X, and about 0.4" shorter than the 030X. Unlike the 056X, it lacks the ball-bearing KVT pivot, 3D machined titanium locking side, and deep-carry pocket clip. This knife retails at $200. One benefit of this knife is the abundant scales available for it.
All of these knives have flippers, and all three fly open, with the 056X knives being extremely fast, and extremely smooth due to the ball-bearing style pivot. All are equally tough, and all are among the most comfortable, grippy, heavy-duty folders on the market. There are some complaints about sharp edges on the 056X knives, but it's no big deal on mine.
The 055X is the shortest, and the 0200 is the longest, though I could not find an overall length in my quick search. All have similar length, and thickness blade wise, and all feel very solid. The 056X and 055X knives do not have re-curved blades.
My pick is the 0561(Brown scaled version), then the 030X, then the 0200. Not that the 0200 knives are the worst, just lacking the features of their more upscale brethren. The 0200 is the budget king for a $160 price tag. The 030X and 056X knives are about the same price, with the 030X knives being much easier to find at the moment.
You CANNOT go wrong with any of these, but for something you plan on carrying often, the 056X makes the most sense because of the diet they put it on, it seems very light for it's size. I'd also be surprised if it's not the toughest, most reliable EDC out there for under $300 too. It's my favorite production knife of all time. I know I'm not the only one who feels this way. I would take it over the 030X price being equal, and it is.
