The Indy4x lines have their DOT approval sealed into the outer coating, so you don't have to keep the packaging. You can see the DOT number here:

http://tad.grosvenor.org/gallery/94TrooperMods/CIMG1963

The line doesn't sit at that angle, that was before I adjusted the little bracket on the axle. They have a more natural position now.

I'm not comparing them on quality as I have never had the Darlington lines, but they are indeed different. The material, construction, (color) and manufacturer are clearly different. I've looked at both up close, and had wanted to upgrade to Darlington lines for a while, but I made the purchase as an impulse buy. I was picking up some tires from Indy4x and saw a set and was very impressed. He has a video of some guy hammering on a line in a vice and it didn't crush.

People with the Darlington lines like them too.