I too own the WR250, the 690 has always seemed like the obvious next step up and if I had the money I would have already done it.
The reason many go for the WR450 is because it can be had cheap used and actually does make for a reliable road bike that can handle a lot of miles.
In the mean time I've just been working my WR250 hard on back roads. I don't like the amount of input it requires to accurately meter its speed over 65mph. Its not that it can't run faster, its that its hard to keep it there. For instance, I might be going 70 and holding the throttle steady. I could look down and have dropped to 60 or gone up to 85 from a change in wind. Point it the throttle requires constant modulation. Some people can naturally moderate by engine sound, I can but only to within + or - 5mph and around here you have to go 7-10mph over to not be run over... and 11 will get you a stiff ticket. Makes the lil 250 horrible on the highway.