Big Dave: I am just starting to use 420V, so, my comments on it are based on specs and data from other sources, not my own experience. I have used BG42 for 4 yeats now, and I have never had a non-bead-blasted blade corrode in any way.
Both of these steels will probably be very similar in corrosion resistance. My spec sheet lists CPM420V at 13% Chromium, but, I believe they just kicked that up a bit, and that is the material I am working with. BG42 has 14.5% Chromium. It is important to remember that you can't base corrosion resistance on the Chromium content alone-it depends on how much of the Chromium is "free"-that is, not tied up with Carbon as Chromium Carbide. Crucible claims that the High Vanadium ties up a very high percentage of the Carbon as Vanadium Carbide ( A VERY good thing!),
and so the corrosion resistance of the 420 V is superior th that of 440V, even though the 420V has less Chromium.
So, my guess is that BG42 might be a little less Corrosion resistant than the 420V.
In either case, if you want to avoid rust spots, you should probably avoid the blast finish as well-particularly sand blast, as it makes deeper craters than the glass beads will. Remember, blasted blases have MUCH more surface area than polished blades, and thus are more subjest to corrosion.
Hope this helps. BTW, my Q-36 has about a 4" blade, and by years end, I'll have some in 420V. Right now, the BG42 is working awesome-all users of my knives have given favorable reports.