Shilen makes good barrels. I know there is not any difference in the accuracy potenial between any of the different designs. What you do get with some designs is a better gas seal, increasing velocites slightly. Also some can be a bit less destructive on bullets with thin jackets.
I have two 1:8 twist ratchet-rifled .223's. One on a .223 Rem and a .223 AI and they are both true hammers. What I was told is the four groove engraves the jacket less in comparison to a six groove improving long range ballistics.
I don't know if the difference is enough to even discuss, but they shoot, and I'm very pleased with them both.
I have a .243 with a #6 Shilen Select Match 1:8tw Ratchet rifling and I am extremely pleased with it. Cleaning is very easy. Accuracy wise it's a hammer. But I never had a Shilen that didn't shoot.