I personally think that they are all great boards! It all boils down to reading the manual and learning how to operate these boards!
Arranger section :
- Roland has the best in my opinion.
(especially drums and bass), hey they make the best electric drum there is (V-drums).
Right Hand:
- Korg , Yamaha
Harmonizer:
- KORG - TC makes professional stuff
OS:
- It is very relative (read the manual and it is easy)
Styles:
- If you don't like what they give you, then make your own stuff + there is a ton available for every single board online!
- In my opinion, all the styles that come with these boards are TOO BUSYYYYYY.
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One advantage that Korg and Yamaha have over Roland is the SAMPLER on top models + multi-pad buttons)
Roland needs to lighten up and follow the trend!
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I CAN NOT COMMENT ON KETRON SINCE I NEVER TRIED ONE OUT!
They are all great boards and require some time investment in order to customize them for your own needs!
Good Luck!