Hi, I have recently bought an impact mk2, but had already decided to buy a barrel kit. I bought it in.22 flavour and the barrel kit in .177 flavour, it didn't really matter which way round I bought them but I do have an Air Arms S510 in .177 so .22 seemed right.
I don't hunt but my reasoning for buying the barrel kit was that for the relatively small extra outlay I got two Impacts for nowhere near twice the price.
During this lockdown period I have been using the .177 for back garden plinking at reduced power and getting 6mill groups with a full 21 rounds and using very little air.
Small adjustments to the hammer spring take care of any changes in power. Wifey bought me the FX chrony for Christmas and that makes checking the power so easy.
If you don't want to go to the extra cost of adding a barrel kit then I would suggest .177 is the better all rounder and I am sure that statement will provoke a few arguments.
Any way, hope my four penneth worth helps.