As far as maintenance cost goes, both should be fairly inexpensive. However, something to remember is that Buell went out-of-business, last year I think. I don’t know if Harley is making OEM replacement parts, or not. I’m sure that there are some aftermarket replacements for the more popular parts. This probably shouldn’t be an immediate concern, just something to think about.
As for the Ninja, kawi is still around and it has consistently been one of the most popular motorcycles for 20 years. So parts will not be a problem for either style, and ‘junkyard’ parts for the old style. For fit, the 2008 and newer Ninjas may be too small for you, as they are more ‘sporty’ than the pre-2008 model, the older model might fit better. However, with different body builds, you might find that they both fit well, or neither do.
Thats my thought on the matter…have fun!