Is there a consensus on proper grip for a bat? I've heard two ways. 1) middle knuckles all in a line and 2) middle of one hand lined up with the back knuckles of the other hand.

How big is the difference between the two?

I don't know if there's a consensus but every coach I've had and the coaches my boys have coach middle knuckles lined up. As they call it "door-knocking knuckles."

The reason being that the alignment keeps the elbows relaxed and is more conducive to keeping the back elbow connected and in the "slot" to drive your hands to the ball and stay inside the ball.

Watch Jim Thome in this video- his middle knuckles line up almost by instinct and it's hard to see but Mookie Betts does likewise. This is a great example of the kind of swing mechanics I'm trying to instill in my boys.

Jim Thome & Harold Reynolds break down Mookie Betts's swing

I think this is a good video about it.

How to Grip a Baseball Bat