My opinion only:
If you already have a .40 cal., and you enjoy it, then get the .40 cal. Kibler kit. You already have all the accessories you will need. As you have said, it will be mostly for targets, fun shoots, and the occasional gopher, so you do not NEED anything larger. I have also found that smaller calibers give a lot more shots to the pound of powder and lead than the .54's and .58's, when you have to pay near double, just for living in Canada.