Barrels are longer on the higher power plant models due to the greater cocking effort. Longer for leverage. With the bigger guns the 125 is base model with synthetic stock. 135 and 155 have slightly larger piston chamber and wood stocks, 135 being a break barrel and 155 underlever.It's the smaller power plant that has a lot of options. Trigger type and spring or vortex ram being biggest differences. More expensive quattro trigger, vortex ram and wood stock makes the top of group then there are synthetic stocks with quattro trigger and the cheaper trigger with wood stock options and all sorts of combinations.
The larger power guns can come in spring version too. Forgot to say that.I've got a Walther Talon (aka 125) in .22 cal and these are nice guns. The reality of it is if you're a squirrel, rabbit hunter and head into the woods for game the lower power plant is all you'll ever need. Being a lighter gun it's easier to carry and easier to shoot. .22 cal 20 fpe will dispatch small woodchucks but if large woodchuck or raccoon is your quarry I'd go with the more powerful guns and .25 cal. There is a surprising number of people with 20 fpe gun in .25 cal. I guess close range it doesn't really matter much. If you shoot larger critters past 75 ft range the bigger gun in my mind is the way to go. The energy of projectile is constantly dropping, the 30 fpe guns are still delivering 20 fpe (foot pounds of energy) at 120 ft distance. And that's the crux of it, the front heavy hard to shoot without support gun for distance you're not accurate shooting distance unless bench rest or over the hood of a car. In my opinion a 95 with vortex piston and without QE in .22 is the way to go. Adjustable quattro trigger and wood stock that not only looks good but balances the weight of gun. I've dispatched plenty of critters with my Walther and really enjoy target shooting off a picnic table for distance to 80 yards. That said, believe the lighter guns are more functional and my purpose of chicken predator dispatch they are close range.