Stiller may have made earlier LP models, that I don't know. The LP Razor looks similar to Stiller's, but Stiller has nothing that resembles the Fuzion. The Fuzion is made in house, and it's a really nice action.
Both are great actions. I agree the trigger hanger of the Impact can be a nice advantage, but if you use a trigger like the BnA Tacsport where you have different sear heights and adjustable sear engagement, you more or less accomplish the same thing.
Since you are having Insite build your rifle (great choice), if you wanted to go Impact they can overcome any obstacles you may encounter with fitment of the trigger hanger in the chassis.
Honestly, you can't make a bad decision between the two. I'm really impressed with the LP Fuzion I have in my hands, it's a quality action. Its not a second tier action like a Stiller (not the Fuzion anyways).