So a 8 year old thread brought back to life.
I think the z250 could tolerate neglect better than a 690. Like on a 690, if you don't check the oil and it burns a bit and then it runs low and you mess up the engine ... is that a 690 flaw? Or say on the 690 the rocker bearings start to wear and the owner notices it doing a valve adjustment and buys the parts and replaces them ... vs an owner that just never gets the valves adjusted ... again, 690 flaw?
So I'd say the z250 is more of a drive it and maybe do a wee bit of taking care of it and no worries if you kind of ignore things, it will still run. And since most owners are more like this, the z250 would last longer for most owners.
And your zx6r should not overheat, so something is wrong on that bike to make it do that.
One thing to consider, a Yamaha MT-07 or FZ-07 (same bike, names changed in the US) ... super reliable, super fun, similar fun as a 690 but needs less attention, the 07 valve adjustment schedule is something crazy like every 24000 miles! Not sure about your area, but in my area plenty of these on the used market with 3000-7000 miles on them.
I picked the 690 because it is lighter than the 07, has better suspension, and a bit more leg room (knees less bent on the 690), and I don't mine maintaining my bikes (kind of overdo it, like I will change the oil at 3,000 miles vs 6,000) ... and if the 690 rocker arms are shot around 15K miles I will just replace them and not get all bothered by it.