The most accurate is a plain steel bull barrel. The more you carve it up, build it out of different materials, etc... the more you can affect the accuracy.
Kidd will tell you their lightweight and ultralight barrels are not their most accurate. You sacrifice a certain amount of accuracy for that light weight.
Longer barrels are both heavier/front heavy and lay on bags with more stability. Shorter barrels are lighter, quicker to point and easier to carry around. If you're mostly shooting off a bench, a plain, non-fluted steel 20" barrel is hard to beat. If you're hunting or carrying the rifle around a lot, a shorter and lighter barrel starts to become attractive.
Choose the actual real life use of the gun, and choose the barrel accordingly.
This being said, Kidd has a reputation for more consistently putting out top notch products. Green Mountain has some good stuff as well. If I were ordering the SAME barrel from both places, I'd feel more certain I was getting a KEEPER with the Kidd. If you're comparing a Kidd lightweight aluminum clad to a Green Mountain Steel... that's a little harder comparison. I have a lightweight Kidd on one of my 10/22's... and it's around mid pack among my 7 rifles... which are all really good shooters, as they have all made the trip to CPC for full tune-ups. I just swapped the CPC barrel on one gun for the Kidd, as I wanted one that was threaded on the end.
If you would send your rifle to CPC for a tune-up, he only works on steel barrels... be it stock of aftermarket.