I have to agree with what Keyman wrote above. You may have seen that table above that tries to compare M4 & M2. I assume it was put together by the people who manufacture M4 because it makes it look like M4 is significantly better than M2. Truth be told, if you took the markings off the blades, I don't think a knife user could tell them apart. I know I can't and virtually every knife I use on a daily basis is either M2, M4 or M390. BTW, M390 is quite a bit different from the other two, but its still on my list of my 5 favorite knife steels.

Never had an M390 either, in what way is it noticably different than the M series?

I know you have to care for M series a lot; not so much if any on M390. M series will get patina, so if you do not want that; you have to take care of the knife and keep it protected.

My Ritters in M2 have a black blade coating on them. It's a older BM coating, BK1 perhaps? Someone who remembers will chime in I'm sure . Anyway, it scatches off relatively easy. But, when they used it on these knives; it was said that the coating, even when scaped off, would still protect the blade from corrosion. I personally have founnd this to be true. I have no patina on these blade at all.
Now, the M4 Ritters have a stonewashed DLC coating on them that I think is outstanding! I've used them a lot and have had no visible scatches at all. I wish BM would use this coating all the time. The M4 Ritters are the only models I know that have it. If you ever get the chance, check it out. It's almost like a bluing on a gun. Becaause of the coatings on the blade, I haven't had to care for my M series blades any differently than any other knives that I own.