Again, it's not really so much of a preference per root, it's more of what works in which location and for the desired effect. Up high in a room, a simple 13 root with a higher overall range works great for slap and flutter echo. Down lower in the reflection zone, I prefer something that will go in a lower range. Either 7, 11, 13,23, etc are all going to perform the same here other than smoothness of dispersion pattern - just a matter that to do a diffuser that will get down into male voice range in a 23 root is going to be pretty big and many people simply don't have room - where a mirror imaged pair of 7 roots may fit and do an excellent job.
Bryan