Despite what the ad says, the cost to transfer is 1.5c pm at the transaction screen, so cost is the same as using the buy/cancel sweet spot.....but without the trauma of the whole LM cancellation experience
But you are stuck with the miles for future use. With the buy/mile option, you do it only when you want to book something in the very near future, so you don't end up holding miles which may be devalued once again or which you might not use for years to come!