Which Caliber Is Better For Under-Lever Airgun

IMHO the pellet does not care how the poof of air gets supplied behind it. The rifle it is going in really does not matter to your question.....the real question is what is a more accurate caliber at 75 yards, 22 or 25. Case can be made for both. The "trick" is going to be mastering that monster spring gun and that is no easy task.

Over all to answer your question, I would like a little more mass it seems to "buck" the wind a little better for me, so I will go 25.
 
You're going to have a lot harder time getting a spring gun to group well than a PCP. In my opinion, this difficulty tends to increase as you get increasingly powerful spring guns, as you have that much more mass moving that much more aggressively inside it. If the name of the game is hunting, there may be something to be said for larger and more powerful calibers, however I personally would be more inclined toward a nice Weilrauch HW97 or Air Arms TX200 or if you really want/need power maybe an RWS54. I just quite frankly wouldn't expect the Hatsan to group well enough at 75 yards to either be useful for hunting small enough game to ethically kill at that range or to be fun to plink with. 

just my 2c. 
 
"Hmmm….I would be interested in seeing the thought process behind that statement."

Don't know that I care to analyze the thought processes behind my observation... as I'm allergic to over-thinking! And you don't really want to know what goes on inside my head, Cherokee. But to shed some light on my purposely-blunt observation, it comes from probably more and wider pellet-gun personal experience(s) than anyone on Earth.
 
I would think that there is not likely much difference in accuracy at 75, but I do have reasons for choosing the .22. There are a lot more pellet choices so you are more likely to find a pellet the gun shoots well. Also the .22 pellets come in a massive spread of weights meaning you can choose a flatter shooting or less damaging pellet if needed under certain conditions. Even with that powerhouse of a springer you would be shooting 30+ grain .25's very slowly meaning a huge drop at 75 yards. If you ever decide to detune the gun with a softer spring you will be glad you picked the .22.


 
One more reason to choose the .22 over the .25 is that even a powerful springer is going to be shooting those .25s fairly slow at the muzzel (650 - 700 fps or so I would think). That may be OK for 25 yards, but by the time they get past 50 I would imagine that they start to destabilize due to the speed slowing down too far. I've had that experience in BR shooting where the 16gr beast pellets shot at around 710fps did fine at 25 yards, but at 50, they looked like a shot gun!

I agree with STO. If you want to buy a springer for hunting, buy a good one like the one's he mentioned and just hunt at shorter distances. I imagine that trying to kill something with a Hatsan Torpedo at 75 yards could result in some badly injured but not killed animals. 

Chas