Rex45 doesn't chip or roll easily, whereas M4 is more prone to chipping. I haven't pressed my P4 hard enough to compare yet, but the increase in edge retention is noticable.
If that someone was me, no I can’t notice a significant difference between K390 and M4. Both are hard and wear resistant. I’d be surprised if most people could in day to day use. It’s good to seem them tested more thoroughly. I like them both.
K390 Urban and K390 Pingo are Italy, I think Italy uses all Bohler steel.
Me too for both, especially the Manix 2. I would quickly spring for a ranger green G-10 model from BBS.
The nicest Paramilitary 2 variant IMO. Be sure to let us know what you think about it.