Tough choice. I'm guessing I even know the maker involved. 52100 will give you about the best push cutting edge that money can buy. It will hold it for a good long time. But it will rust fairly quickly if you don't do maintenance from time to time.
420V (now renamed to S90V) is a really fantastic small blade steel, taking a great acute edge, holding it for what seems like forever, and acting every bit like the 'stain-less' steel that it is (having more than 12% free chromium in the alloy mix). I have small folders from one maker in both 52100, 420V, and 12C27. They all work really well as cutting tools. Each steel has its advantages, and disadvantages, as well as cost differentials.
52100 will produce probably the best edge and is relatively easy to resharpen, with 420V running a close second in edge quality, but a clear first in edge holding. It is a little more trouble to sharpen and a lot more difficut to finish, which adds a lot of cost. But it acts like a carbon steel but has plenty enough chromium to be stainless, without compromising edge performance. So you can cut an orange and not worry about tarnish. That is a Very good thing for Some knives.
For me, if that daily carry folder is going to be a letter and fruit knife, with an occasional box cutting chore, go with 420V and don't look back. You won't ever need to worry about it discoloring or rusting provided reasonable care, and it will work very well.
If you want a really tough, really superior cutting tool, and are willing to pay a little less in manufacturing cost, but more in maintenance expenses, go with 52100 blade steel.
Either steel will serve you very well indeed. It's entirely a personal choice, and depends greatly on the knife in question. So I decline to state a preference based on a lack of information and the fact that I am not YOU.
Paracelsus