I guess I'm probably known as a Cain "fanboy", having written three articles about his classes for SWAT, but I agree with you that Cain first, USA second. I have trained with Frank and Shannon from USA, and shot with Manny in matches and classes (a Pat McNamara class we held at their facility).
Randy is going to teach you Weaver. he's going to teach you to ease to reset. He's going to teach you a hard front sight focus all the time. He teaches what he was taught, what he knows, and what works for him. But if you show up wanting to shoot Iso and you can do it well, he's not going to fight you on it. I, perhaps obviously, don't see anything wrong with this. I think it's good to start with the "old" way because Randy is fantastic at teaching and explaining the fundamentals that way.
I have yet to find a "post-modern" Randy.
The USA guys are great teachers for what they are, but I think you want to have a good grasp on the fundamentals from elsewhere before you go there.