Hi, as I have had an ER6N in the past and picked up few days ago my z650, I might add my two cents.
i am a small woman rider (1.63 m about 5 feet 4?).
I loved the ER6N but it was hard on my wrists as the sitting position was a bit too forward. Might not have been the case with taller riders with longer limbs.
With the Z650, the position, from my perspective, is much better. I rode it 2 hours from the dealer and had no issues with my wrists, so the position might be a little more upright. None of the bikes felt cramped, but then you would if you are tall.
i remember the ER6N felt maybe heavier and also a bit taller?The z650 feels incredibly nimble, light, I can put both feet on the ground at the same time (! Makes a change from my usual tall touring bikes).
It is so much fun to ride ( like the ER6N). It is very fast, I found myself at 80mph on the motorway while overtaking a truck before I realised.
All in all, the new version feels better for my size than the ER6N. I would definitely recommend for short riders.