You dont mind the riding position, from sitting on them, or riding? If you sign up for Motorcycle Safety Foundation class, they usually have both styles available. You can ride all day and make SURE before you buy either one. The sport bike will be uncomfortable on your neck after a few minutes, until you build up core strength to ride sitting up. And at 6', the rebel may feel cramped after an hour or so.
Both are reliable bikes. The problems in these 2 used bikes are from beginners buying them, not being familiar with how do do maintenance and neglecting them, or quit riding and let them sit. I see a lot of both of these for sale in my area, with the Ninjas being more beat up then the Rebels. Young folks like to try stunting, get wrecked or scratched up. New, they would be fine if taken care of properly. ( I personally dont advise anyone to buy a new bike as their first bike though.Take an experienced person with you to look over used bikes.)
Depends on what kind of riding you plan to do also, short commute to work or school, all day exploring? Ive seen 2 up on both bikes. They have different gear ratios, sport bikes for quick starts, cruisers for maintaining higher rpm for longer time.
Not sure this is what you had in mind as an answer, shrug, but my .02