What are you going to do with it? If you intend to get into serious target shooting such as Benchrest, then pick another caliber-.177 is best. .22s can be finicky unless you spend lots of money for custom barrels.
Agreed, it all depends what you are going to be doing with it, backyard stuff .117 call. Best all around for 10-100 yards (plinking, target, small game) would go with a .22. .20 is a non option due to limited ammo selection. Just know that if you stay in the game long enough you will get a .25 eventually. Happy shooting. I took my tax check and got a .25 Tactic (still waiting for it).
I think the .20 got popular with spring rifle shooters simply because the power limitations of most spring guns. If you were shooting a .177 your shots were coming out faster and you had a flatter trajectory. If you were shooting a .22, which has more knock down power for hunting than a .177, your shots were coming out slower and you had a more loopy trajectory. I think the .20 was just meant as a compromise between the two.
With the power of today's PCP that is not a compromise that has to be made any more. The only advantage with .20 that I can think of is that you might get a few more shots due to needing a bit less air. The difference however would not be worth it to me lose the selection of ammo that is available in .22 and the .22 will have a better balistic coefficient and be better for longer range shooting. I would take the .22 and not look back.
If you limit yourself between 20 and 22 go with the 22 as the 20 is going away. If you even think of doing some hunting or pesting go with a 25 cal. Unless you are a Beeman springer junkie I just do not see any reason to go with a 20 cal.
I'm with Ginuwine 100% . It is very hard to beat a 25 . Pellets readily available, great knockdown power, quiet,and most are efficient. I sold my 25 a while back , and kick myself in the ass every time I think about it