I can't say which kit is the best because I've only used J&T. But I can say that J&T's kits were entirely satisfactory. But-- if you want a chrome-lined barrel, the only option with a J&T kit is their M4gery, which has an integral muzzle brake that some people might not care for. If chrome-lined isn't a big deal for you, or if you're looking for a heavy stainless varmint gun, they're great. I've built three and I'm completely satisfied.
If you buy a complete rifle, you might get a better match on the upper and lower receiver finish, but I don't think the quality is any better. There's not a whole lot to assembling an AR from a kit (the upper comes fully assembled with J&T kits), so as long as you're getting quality parts, I don't see why having it factory built would make it any better.