What chord do you usually play as the second chord in bar 6? Take a Bb progression and in bar 6 do you play Ebmaj7 - Ebmin7 or E dim7?
What's the most common etc...
But then again I generally wouldn't play Bb-Gm7-Cm7-F9 through the rest of the changes either. Sometimes I'd play Dm7-Db9-Cm7-B9 (Cb9). Or Bb-Db9-Gb13-F13.
Depends on what other instruments are playing. Sometimes it would be 2 or 3 note partials.
I've seen both Ebm and Edim7, probably about equally. It depends on what line you want through the chords. Edim7 gives you a rising bass line in bars 5-6: Bb-D-Eb-E (and then to F on the next Bb). Ebm gives a descending line, usually in a higher voice: Bb-Ab-G-Gb (leading to F from the other direction of course).
Ab7 is rarer, and would be a sub for Ebm. (bVII is quite a common sub for a minor iv, but I've not seen it much in rhythm changes. Sounds good, mind.)