The problem at 2 years old is the roots may be too stiff to really follow the contours of the rock. ROR looks best when there are no gaps between rock and root and that is difficult to get with older roots.
As mentioned, Trident has faster growing roots so much better. JM can work but is best with a lower rock and will take years for the roots to grow well and hold the rock well.
I find 'butterfly' is less tolerant of dry than most other Japanese maples. Any bonsai with rock in the pot dries out quicker than normal pots so be prepared to water more or put up with burnt leaves in summer.