The Infinity has a single air tube whereas the III 300 has two, so it probably weighs more. The III 300 has open sights, the Infinity does not. It isn't mentioned in this writeup but I swear I read somewhere that the Infinity has a shrouded barrel, can neither confirm nor deny that. I know that the pictures on Pyramid show different stocks, but "Indonesian Walnut/Hardwood" is pretty much the same on all of these guns, at least the few that I've seen. I would say the biggest difference is the single vs. double airtubes, that is what you are paying an extra $100 for.