While I do not have an 0620, I do have a Bad Monkey that I love dearly. I've owned many ZT knives in the past and still own one now (very early 0350), but I have to say that the BM is in a slightly higher class from a F&F standpoint. Overall I would have to say that Southern Grind's QC policy is a little stricter, not to say that ZT's is bad by any means, just that it is apparent that the ZT is a straight production class knife whereas the BM approaches a more mid-tech classification. Just my observations and opinions. Also, with the Bad Monkey, you are getting a more unique product, a knife that doesn't really look like any other from a company that is very passionate about their products. I assure you that Zac Brown did not start Southern Grind because he wanted to get rich.
That being said, it is true that the steel choices that they have respectively used do set these knives apart for sure. While it would seem that a comparison between M390 and 14C28N would be an exercise in futility, they do share some things in common. I own a few knives with M390 (Benchmade 480-1 and 755MPR) and EDC my Bad Monkey fairly often and use them to the point of needing to be sharpened regularly, so I've been able to form some opinions here as well.
Obviously the biggest difference between the two is edge retention, where M390 is undoubtedly superior, but SG's 14C28N is certainly no slouch. From what I've read and observed to a degree, Southern grind has figured out a very good heat treat for their steel. It is very strong and will hold an edge for quite some time. I can break down heavy cardboard boxes all day and still slice paper cleanly afterwards. The difference between the M390 and the 14C28N is that I can do that and go 2-3 days between sharpenings with the M390 whereas the 14C28N I usually end up needing to sharpen it by the end of the day. Either way, the 14C28N is a workhorse and I have no concerns about it letting me down 1/2 way through a job.
They are both very corrosion resistant so that is a non issue.
I find it easier to sharpen the 14C28N than the M390. I never let my edges get very dull. If they won't shave arm hair fairly easily, then they are ready to be sharpened. I find it much easier to keep my knives extremely sharp if I regularly maintain their edges and never let them get too dull. With the BM, a couple of passes on the Sharpmaker brown rods and then the white rods and it's hair-poppingly sharp. And I literally mean a couple passes, The Bad Monkey sharpens up very quickly. The M390 will sharpen just as well on the Sharpmaker, but it takes more than a couple of passes. With the M390 I need to invest a little more time with the corners and flats on both the brown and white rods.
I haven't noticed any chipping with either steel, and only slight rolling when hitting a staple (breaking down boxes) or sometimes when I'm careless doing something stupid around the shop (like trimming a wooden gunstock and slipping and hitting the steel receiver, I have done this on more than 1 occasion).
So, I guess in summarizing, yes, M390 is a better steel, but the 14C28N on the Bad Monkey is VERY good in it's own right. By no means is it the same blade that you would receive when you buy a Kershaw, not hardly. With the Bad Monkey's heat treat and blade geometry, you get a very strong blade that sharpens easily and will work all day long with minimal maintenance afterwards. Trust me when I say that if I wasn't completely satisfied with the performance of the steel on the Bad Monkey, I would not own the knife. Not only am I satisfied, but I intend to keep that knife forever.
