Unless your partner likes to open 8 counts you know you are going to slam and given that the system does not allow you to find out more about partner's shape we are basically only interested in ♠K and ♦AK to choose which slam. It seems logical to use whichever is the most efficient method of achieving this. Out of interest, what would a 2NT rebid mean? If this were a shape ask, for example, then this could easily be the best option (especially as partner would misinterpret it).