With the adjustments on the Avenger, it really doesn't make a lot of difference to most shooters. Accuracy is pretty much the same in both calibers...depending on whose's shooting.
However, if you need knockdown power, and want to go the limit on fpe, then I'd suggest the .25 with slugs or heavy pellets. Depends on where you live, larger game in some states.
If you shoot occasionally, then the .22 will still give you plenty of punch and accuracy, is a tiny bit cheaper to operate, with a higher shot count per fill.
I favor the.25 myself.
Kindly 'Ol Uncle Hoot