I'd say BK7m hands down. You can use it with your own keybed (which would be always better choice than the bk5 keybed) and it can plug into anything, single channel, multi channel, even midi guitar or accordion. You can't do that with BK-5, you can't even address the arranger from external midi device so once you have bk-5 you are stuck with its keyboard.
And it got 1.06 firmware finally.