Proud owner of a GW-5600j here but my eye has been wandering to the GW-M5600. Advantages that matter to me are day on the display and the slightly smaller case which may feel better on my 6.5" wrist. Downsides for me are the price and the red border and excessive text on the face. I go back and forth on whether the red border makes it look "cheap" in comparison to the 5600j or whether the red border gives it that extra pizzaz. Also, seems like the GW-5600M has a lot of info on the display. A bit cluttered versus the GW-5600j. Tough to decide without seeing one in person.
I am a one watch kind of person. That is, once I like one I tend to wear it all of the time.
So - what do you think? Seems silly to be asking others about what comes down ultimately to my preference but curious to hear from anyone who has both or maybe one or the other.