Kral slug rifles in .22 - What should we choose?

Good day guys.
I need to lend some knowledge in a buying decision.

I have been trying to find information and reviews of two Kral guns which I have to choose between. One being a Super Jumbo and the other a Bigmax X (both chambered .22 caliber). And I am simply struggling to get my self informed to make a choice as there isn't much content available on the two guns.

My friend who owns an air rifle store keeps pushing me into getting a Super Jumbo which he will modify by swapping the 535mm barrel for a 580mm barrel and possibly putting in a tighter hammer spring. I don't think he wants to try and regulate it. The reason for these guns are my use of slugs. I have no problem with modern pellet projectiles, but simply prefer the harder hitting slugs as I take larger game on the farm and keep to humane practice.

I know the Bigmax X can chuck a 25.39gr JSB knockout slugs .217 at 900fps which gives about 58ft/lbs out the box. And for my intents and purposes it is very accurate up to 100m. But my friend insists that the modified Super Jumbo will do better.

The aim is to be able to ethically and precisely varmint and hunt small game up to the size of a guinyfowl or jackal at 100m and be able to take spurwings and monkeys with head shots at 50-100m. Again, slugs are a must for me. And my budget on this hobby/sport allows for these guns.

I hope to hear from you.