I think I can give a fairly good advice being that I own both of these MC.
I started on the Ninja because I am a short gal and I needed a shorter bike without too much power as my 1st bike. The ninja looks good, it drives nice, parts are easily available for it, easy to work on too with the canter stand. Some people prefer liquid cool bikes and so I guess this could be a plus as well for the Ninja. It proved to be very reliable since begining on the market 20 some yrs ago. Basically you cant go really wrong with the Ninja.
On the other hand, after passing my MSF on it I sold it to get my Buell Blast. why you must ask? I fell in love with the Blast, period! I never felt that way about my Ninja but this is subjective and a very personal opinion.
the Blast is just a better bike for ME. it sits just a little bit lower, which is good for me, it is a 500cc so a little bit more bike but it wont go any faster than the Ninja or at least no enough to justify to exchange but then this is not why I bought the buell. The buell VIBRATES like there is no tomorrow but it doesn't really bother me, it is worse at lower RPM but it runs like a dream after 45-50 mph. I think the bike sounds awesome! the Ninja has that very high pitch sport bike sound that I dont really care for. The gears on the Blast, are a noisier, specially the 1st one when leaving from a stop. I'd say that the Buell is somehow of a rougher bike, as oppose to the Ninja which I think is more delicate, if that makes sense. A more basic machine is maybe what I should say.
I also like that the Buell fairing are not painted, they are made with a DuPont material called Syrlun, which is used to make golf balls, so the color is pigmented, not put ON it. It will never fade or peel and it there are scratched, you can just buff them out and it will look brand new.
The Buell is a naked bike, not as much heat coming off of it as the Ninja that is a full fairing bike, that also makes it less expensive(Blast) to fix in case of a drop. The Buell goes through back tires and oil more than the Ninja but it has a kevlar belt instead of a chain, no adjustments need for the Buell. The buell comes with an automatic choke so no need to mess around with that on a chilly morning, start her up, it will warm up just fine and be ready to go right after you're done putting on your helmet and gloves
they both get very similar gas mileage too, 60-70 mpg. Insurances were pretty much the same for both bikes but then I am a 38 yrs old female so it is cheap for me...LOL
You can buy them used for about the same price . The Ninja will probably resell faster than the Buell.
in conclusion, both are great bikes, excellent to start on and then some. Best advice is to go sit on both and see how it feels, the Ninja as a more sport bike sitting position but nothing extreme, the Buell is what's called a "standard" bike so you sit upright, how comfortable the bike will be for you will be the best indicator of which bike you should get.