I have P35s in 177, 22, and 25. All have killed at least 10 squirrels. The 177 tuned to about 18 fpe is plenty within around 35 or 40 yards. It is very quiet with a printed moderator in the shroud and gives me nearly 100 shots on a fill. The 22 is tuned to a bit over 30 fpe and squirrels hit with it drop a step or two quicker than with the 177. I get at least 60 shots on a fill from it. I retuned my P35-25 to about 50 fpe and I'm not sure about the shot count. I used it to take 18 squirrels when it was tuned to about 32 fpe and at that level I got at least 70 shots on a fill. It was very effective at 32 fpe, I just decided it should be higher than the 22.
The P15 is pretty much the same gun with rear cocking and a wooden stock. I think it is also a bit lighter. If the rear cocking doesn't bother you it may also be a bit less expensive.
The key benefit of these guns is they only weigh about 5 lbs but still offer plenty of power for small game and a reasonable number of shots. I also shoot 30 yard challenge targets with mine and the P35-22 gave me a 200 last year. All 3 are accurate.
The P35X has a wood stock, a bit longer barrel, and a 60cc plenum. It would be a good choice for a P35 if you want a 25.
The M60B is a newer gun and a different style gun. With air storage in a cylinder it isn't a lot more expensive than a P35. With a bottle it's more but still only about $700. It's heavier but the barrel is longer. You can get it in 30 and 357 caliber. It can be a more powerful gun.
The M60B and the Avenger have regulators that can be adjusted externally. That is nice but I can change the internal regulators on my P35s in under 15 minutes. But I have to degass to go either up or down. On the Avenger and I believe the M60 you only have to degass to turn the regulator down. That is an advantage. Both also fill with a foster fitting instead of a probe. I think that is also a not insignificant advantage. The M60 air storage chamber (bottle or cylinder) and be removed charged. So if you need to degass you do not loose all that air. I don't think the Avenger will do that.
I have an Avenger but I would get one of the SPA guns. I think they are better built. But my Avenger is not the newer Avenge X.