Not to be too argumentative, but the Ninja 500’s engine is 20 years old, just like the Katana. It has been used on several bikes (ER-5 series in europe, including a dual sport).
Yes, it has coolant that needs to be replaced every two years, but that really isn’t a big deal – cars manage.
The Kat has more complicated fairings, which adds time to the shop labour costs.
And while the valves are screw adjuster (pretty easy to do) there are four sets, versus the Ninja’s pair. The big question is how often do those valves need to be inspected and adjusted?
I can’t say. I can find a maintenance schedule for the ninja on EX-500.com, but I can’t find something similar for the Kat.
But I think the fact that the bike will primarily be a city runabout is really important here. Riding any air cooled engine in traffic is uncomfortable. It can really cook your leg on an already hot day. The Kat’s fairing are supposed to be pretty good at deflecting that heat, but they aren’t perfect. And riding a 480 pound bike in the city is a very different game from riding a 400 pound bike in the city.
I’m not calling the Kat anything but an awesome reliable sport touring rig. I’m just saying don’t discount the Ninja 500 as unproven, unreliable, or maintenance heavy, because it is none of those things.
As an aside, maintenance costs are a really good thing to look into when buying a bike. My ZZR-250 required valve adjustments every 5000kms (once a season for me). This maintenance cost me $300 at the dealership. My new bike, an SV650N, doesn’t need valve adjustments at all. In fact, it’s maintenance schedule is basically just oil changes (which I can do myself in 20 minutes for $45 using synthetic). So, despite the fact that my ZZR-250 would cost me $300 more per season in insurance, the maintenance savings nearly balance that out. I like it when things like that happen