I also have the Costco set, it's pretty complete and a good value. I dont have a problem with the impact, I've never put a snapon to the test but I know the Costco one is still doing well over 100 ft-lbs, a range that when I get up there I'd rather pull out a torque wrench and get it right (except lugs nuts, I have my own method of using the impact on those that works nice.) Go for the Costco Kit and if you find the impact insufficient then get a better one but you wont regret having the rest of the stuff, it's well enough built.
-John

85 4Runner, 4" lift, bumpers, sliders, locker