I've really liked my kx450f, and will always have my k5 around, but for the riding you're talking you'll be better off. I get barely 20 miles to the gal on my k5. All the bad stuff that happens to 4 stroke is greatly reduced if you're not running in the high rpms like racers have to do.
I was hoping Kawasaki would stay with the klx and add the efi. I guess sales tells it all, but they aren't making a whole different bike only a handful of changes from the kx. Maybe if the economy turns around they'll revisit. I should be in charge of Kawasaki marketing in the US,,, there'd be a few different dirt bikes and would include 2 strokes,,, and all with efi.