Ray , I have hired both the ER6N and the Ninja 650 from Mrs Mechanic in CNX and ridden the MHS loop on separate occasions. The ER6 I rode solo and the Ninja 2 up , with my wife on back. I really liked both bikes.
I also own an ER6n here in Phuket. It is a 2010 model with ABS and can be bought easily around Thailand. The Ninja is more expensive due to all fairings etc.
If I understand your situation correctly , it is your dodgy back which is the big question ?? I also have a crook back and personally, find the riding position great for my back. Better than the upright positions of all my previous dirt bikes which perhaps seems a bit weird.
I once had a Phantom which I found very unsupportive for spine position.
I think the 650's are a great mid range engine giving good power but downsides are poor visibility in mirrors, clunky gearbox esp. neutral to first ( I'm used to Hondas) and suspension a bit firm for me ( I'm 67 kg )
At speeds up to 110 - 120kmh , the ER6 is fine, but if you plan to do longer distances at sustained higher speeds , I would suggest looking at the Ninja.
Hope this helps