Both great knives, both worth getting if you like them IMO.
The Demko is the new hotness, but the PM2 is an icon (and for good reason). The PM2 is definitely a better value for your money by far when it comes to materials, which I assume is the dilemma from the framing of the original question. I'll not use the word overpriced with the AD20.5, because I believe markets set prices; and I think Demko is simply reaping the benefits of his intellectual property, which in this case just happens to be an innovative (and very cool) locking mechanism that many seem to be clamoring for. Good on him if you ask me. Can't fault a man for getting the bag while the getting is good. Just know as a consumer what you are paying for. As I see it, with the 20.5 you are paying for the Shark Lock and the time invested in developing a new knife and getting it to market, whereas with the PM2 you are dealing with an established product from a longstanding company that has the ability to focus better on value when it comes materials (not to mention decades of experience making superb cutting tools).
I personally bought the 20.5 as a stopgap to experience the Shark Lock and hold me over until (or if) the perfect configuration of an AD20 presents itself to me at the right timing. I'm happy with my purchase. I don't currently own a PM2, but I probably have more Spyderco knives than any other brand, and I don't think you can go wrong picking a Spyderco in whatever configuration you think will suit you best.
TLDR version....Get bof