It's all preference honestly. I have never experienced the straight tube moving and it fits so tightly I don't ever see it moving unless you want it to. The flared one basically guarantees that the shim cannot move any further into the frame, and provides a nice rest for the return spring to go up against. The only downside to the flare is that it is visible where as the straight is completely hidden.
This shows how the flared tube looks when installed: