Personally, I find the 250 class air cooled machines a little underpowered if you have ANY intention on doing freeway miles. Sometimes you need a little reserve power to help get you out of trouble. On secondary highways and such, they're fine.
I have a WR250F which is the enduro bike, which I have blue plated. I've had it up to 80mph and it was still pulling. You'll run out of guts or revs before it runs out of power. I got this because I wanted to do some enduro and hare scramble competitions. It's street plated, but street use is pretty limited. It's like taking a thoroughbred for a trot.
The new Yamaha WR250R and X are nice bikes. They have much more power than the older air cooled machines. Those are a great option. Otherwise, I'd recommend the DRZ400S. But, if I was going to do that, I'd just get a DR650 instead.
It's really like the AT/MT tire debate. There is no perfect compromise bike. If you want to be competent on trails, you'll have to settle for a bike that is incompetent on the freeway, and vice versa. The only possible exception is an advanced 250 like the WR250R or a KTM. And don't forget, it's not just the power that's at issue. Comfort is a big part of it. Most of the higher performance bikes have narrow seats. You don't want to ride a razor blade for 2+ hours. The bigger bikes are really heavy on the trails.