To me... both involve the same tasks... retrieving a fresh mag, replacing the partially expended mag, and then securing the partially expended mag... both offer a solution with advantages and disadvantages.
Each has the rifle in a 0ne-shot posture for a very brief period of time. It seems Keller's way is more deliberate and w/o delay... but... creates a period of time where you do not have physical possession of the partially consumed magazine...meaning you must remember/make sure you secure it... otherwise, should you get flushed from your position or get distracted, you risk leaving something behind for the enemy or you may end up needing those few rounds.
All hypothetical of course for most people, but a serious consideration for a lone cop dealing with an active shooter.