Ditto. SV650 is an amazing bike. Good for long trips, track, city, commuting, you name it. Plus, once you get used to the V-twin torque, you'll love it. It is a light and nimble bike. Only about 30lbs heavier than Ninja 250 and actually lighter than Ninja 500 (which would be a good bike to consider as well if you don't mind the outdated look).
You can find SVs cheap, insurance is inexpensive, maintenance - low.
FZ6 is much more powerful, not THAT far off an older version R6 from which the motor derives, it's also a larger, heavier bike.
FZ6R is more like SV650 and Ninja 650 in terms of power delivery. A little less on low end and a little more on high end.
Having said that and coming back to the original question - definitely Ninja 650 over 400. Ninja is exactly the same as its bigger brother but with a castrated engine for insurance purpose in BC and Quebec. No point wasting money on that bike.
You can also consider Honda CBF600.