Forgetting the fine print of the code, it's good practice to have all the wires that have anything to do with a circuit wind up in the same panel. It's also nice that nothing go through the feeder conduit but the ... feeders
Keep in mind, regardless of the distance between the panels, or the method of connection ... always run the necessary four wires. Depending on a nipple, a screw, or any other hardware for uniting the grounds is unreliable - and if you try that with the neutral (heaven forbid!), you're setting things up for a major failure down the line. (Visit the chat room if you want to hear how I spent my Thanksgiving!)
