Just in the last few days, I've been tweaking edges on a couple of knives; one is a Case '75-pattern large stockman ('Tru-Sharp' stainless, a.k.a. 420HC) and the other is an older Camillus-made Buck 311 slim trapper configuration in 440A. In sharpening these, I'm reminded once again how similar the two steels are, at least in terms of how they sharpen up, and the edges they can take. Both are pretty similar in carbon content (440A might be a tad higher), and the 440A also has more chromium than 420HC. In terms of sharpening, I used the same means for both knives (I was fiddling with a Norton Economy stone in SiC), thinning the edge grinds on the Case stockman's clip blade and on the Buck's spey. Followed by stropping on my denim-over-wood strop with some aluminum oxide compound (Sears #2), with some light finishing touches on a leather belt with green compound. Both sharpened up just as easily, and both took nice tree-topping edges in the process. I have an older Camillus-made Buck 307 stockman in 440A, and I'd seen similar behavior in that one when I sharpened it up quite a while back.
Hard to say which is more 'durable', as that depends on a lot of factors, such as heat treat (hardness) and blade geometry. In typical uses, I've never noticed any significant difference in any iteration of 420HC or 440A, in terms of how long an edge lasts before needing a little touching up. Anymore, I tend to favor steels that will take very, very fine edges (testament to fine grain, a result of quality-sourced steel and good heat treat), which makes them very nice slicers. Both of these steels handle that with ease. Even if they do need a little more frequent touching up, either of these steels makes that very, very simple. Very easy to live with.
David