Unbiased owner review of these models, but not the caliber... I have both a gen-1 Impact and an Evol both in .30 caliber. We Impact owner's have helped evolve the unit into the thing that it is today. I have spent more money and time on that Impact trying to get it up to the Evol's standards than if I would have just bought the Evol from the beginning. The AAA Evol is not a gun that needs evolving, as it was engineered correctly from the beginning, based on what was lacking in every other airgun manufacturer. It is an expensive piece of kit and there are not a lot of people who are willing to shell out that amount of coin. The Impact has a higher following because it is marketed much more and the manufacturer has been around for a long time. But I'd buy another AAA product in a heartbeat over the Impact now.
As with any slug shooter, you need to figure out which slug weight, length, diameter and design shoots best out of the barrel. As an Evol owner, you would need to find the slug the barrel likes. As an Impact owner, you could simply buy the proprietary slugs (the FX Hybrids) that we owner's helped fund the evolution of in the Impact. Keep in mind too, the Hybrid slugs are for pesting varmint and birds. This is what the Impact was built for. The Evol is a game hunter's tool, and can handle big game as well as small with the right slug selection.