The thing that you miss out on not going from the BK-7m up to the BK-9 is quite significant, IMO.

Primarily, the Chord Sequencer (not in the BK-5/3/7m OS yet, if ever), and the audio key feature, to play audio loops in sync with the arranger. Just those two things alone push the BK-9 out of the MOTL market and into the TOTL segment, especially in comparison to the other BK's.

The B3 emulation clone is also a massive improvement over the lower BK-s. SN (SA type sounds) as well, and a boatload of other stuff, but as a BK-7m owner, I could not pass on the BK-9 when it came out, despite much of the sound and style set being similar. Those features made it a must buy.

Sounds are sounds, you can work around most shortcomings. But FEATURES, especially radical ones not available on anything else, that's what makes something a must buy, for me...