I agree: Ketron first, and probably Korg second. Ketron definitely has some thoughtful and imaginative customization options to authentically reproduce specific genres of music! [What I don't like about Ketron is their chord RECOGNITION. It tends to acknowledge sixth chords over the more common minor 7ths, and I'm not about to re-learn fingering for songs I already know how to play!]

Yamaha and Korg have similar parameters, which is evident by how well Yamaha styles convert to Korg format. But the two companies take different approaches. Korg gives you access to everything. While Yamaha does allow different patterns for major and minor chords, this option is hidden, controlled by parameters in the CASM that can only be accessed with 3rd party software.

I would rank Casio fourth. They have a pretty complete range of control options. However, none of their arrangers since the MZ-X2000 have had a "memory" parameter to mute the accompaniment but keep the drums going when you release the keys. (Sync stop kills both.) The factory styles are dull, and except for the newest AiX chip, the sound engine isn't great, which doesn't help.

I'm a huge Roland fan, but for style control I rank them absolutely last. They don't even have a 'retrigger' parameter! Or at least it's undocumented and possibly implicit in the sound sample. Likewise the wrap/range; again I believe this is determined by the selected instrument sample. Frankly, I'm amazed at just how playable and good-sounding Roland styles are, given so few options in the Style Composer. Alteration mode was added in G-70 version 2. (This is similar to Yamaha's Note Transposition Rule, and determines whether a chord is revoiced "close" or parallel.) But at the same time they mysteriously dropped "chord family assign," which allowed a user to assign one of the recorded variations (M, m, 7) to a given type of chord.

Another thing that frustrates me with Roland is that they don't give you an option for "no transpose." Multipads finally arrived with the E-A7. But phrases ALWAYS transpose to match the played chord. You can copy style patterns to the pads, and use them as accompaniment to play over a non-transposing MIDI song. But doing so takes a lot of time and effort; a primitive workaround for a feature the competition's had for a long time.

One thing that jumps out at me after studying many reference manuals: All the brands had 90% of their style format (and related user parameters) in place 20 years ago, with only minor additions since then. There truly is nothing new under the sun!
Good topic!!