How much money do you have to spend? A 650 dualsport will be around 2500-4500 used for a really clean machine, it will keep up on the highway and keep you entertained on back roads and dirt. 50mpg, cheap insurance, and on a tight bumpy road positively embarrass sport bike riders. The KLR will be nicer on the street, the DR better in the dirt.
If you have no inclination to ride single track dirt trails, but maybe forest service roads, but mostly street, look at a Suzuki DL650- this has a v twin motor, a fantastic all around motorcycle. Far more capable on the street than the singles, but still capable on the dirt roads. Has some wind protection, often folks replace the windscreen as the factory one can cause wind buffeting- An ideal all around ride- about 4500-5500 for a super clean example ,used. Also will return about 50 mpg.
The little Ninja would be fun for a bit, but really limiting - it's nice to have a bike you can jump curbs with, go over BFPH (big pot holes)
or play off road.
In general, if buying used, my recommendation is to find as stock a machine as possible with regard to engine or exhaust mods.
Look for stuff like an upgraded shock, braided SS brake lines, better seat , etc.
