“…Katana is a better bike that’s will last a long long time over the Ninja 500.”
I think you’ll find both are famous for going for pretty much forever. The Ninja 500 is an incredibly reliable water cooled twin. The Katana, is an air cooled four.
The Katana is more expensive to own. It drinks more, insurance is more, and costs more to maintain (4 cylinders vs 2, lots of fairing vs just a little to remove, bigger more expensive tires). It is also famous for its vibes.
At 135 miles, the Ninja is brand new. At 8k the Kat is not much more than broken in.
Both are great bikes.
I think it really should come down to a matter of comfort for you. The Kat is a nicer highway ride, and will “keep up” with the big boys a bit better. If those are your goals, it is the way to go. If you like the lightness and narrowness of your 250, but want some more power, the Ninja is a steal. On public roads with considerations for real world things like road conditions and traffic, it isn’t significantly slower than the Kat, but will feel much lighter (the Kat feels like a big bike, at least to me). If lower running cost or working on the bike yourself are important to you, the Ninja is the better deal.
You’re chosing between a V6 Mustang and a V6 Camero. Both are loads of fun, which you chose only comes down to personal taste – and either way the “real man” on the “real” ride is going point and laugh (at it’ll be entirely to his loss).