Differance between 420 and 440 stainless steel

420 stainless steel
Lower carbon content (<.5%) than the 440 series makes this steel extremely soft, and it doesn't hold an edge well. It is used often for diving knives, as it is extremely stain resistant. Also used often for very inexpensive knives. Outside salt water use, it is too soft to be a good choice for a utility knife.

440 A - 440 B - 440C stainless steel
The carbon content (and hardenability) of this stainless steel goes up in order from A (.75%) to B (.9%) to C (1.2%). 440C is an excellent, high-end stainless steel, usually hardened to around 56-58 Rc. All three resist rust well, with 440A being the most rust resistant, and
440C the least. The SOG Seal 2000 is 440A, and Randall uses 440B for their stainless knives. 440C is fairly ubiquitous, and is generally considered the penultimate general-use stainless (with ATS-34 being the ultimate). If your knife is marked with just "440", it is probably the less expensive 440A; if a manufacturer had used the more
expensive 440C, he'd want to advertise that. The general feeling is that 440A (and similar steels, see below) is just good enough for everyday use, especially with a good heat treat (we've heard good reports on the heat treat of SOG's 440A blades, don't know who does the work for them). 440-B is a very solid performer and 440-C is excellent.

Now, since 420 stainless steel is softer (and also less brittle than most 440), it might be a better choice for an axe or throwing dagger, but I would say it still wouldn't be as good as a simple 1050 or 1060 carbon steel.

So, in summary, 440 stainless steel is more than one steel, and is not just for decorative blades, and 420 is not neccesarily great blade steel at all.