If one of your high priorities is gas mileage, that might rule out the DRZ400S. The five-speed vs. six-speed issue is possibly not quite as big an advantage for the 225 as some people think. What Yamaha has done with the new 250 is spread the gears out farther, so there is more of a gap between each gear. The 250 isn't quite as wide from low to high as the 225, but it's close. How they do on the highway depends on vibration as well as gearing.
The closest thing to a direct comparison I've seen is from Duckspanker. You might want to PM him for more information:
As far as gear ratios go, here's the comparison, widest to narrowest, of some bikes in this class. Note that the 5-speed Yamahas have wider gear spacing than the six-speed KLR or KLX 250s:
3.90 Yamaha XT225 (6 speed) 19.69
3.78 Honda CRF230L (6 speed) 19.12
3.63 Kawasaki Sherpa 250 (6 speed) 18.30
3.46 Yamaha XT250 2008 (5 speed) 17.50
3.45 Yamaha TW200 (5 speed) 17.43
3.36 Yamaha WR250R/WR250X (6 speed) 16.99
3.32 Kawasaki KLR250 (6 speed) 16.76
3.15 Kawasaki KLX250 (6 speed) 15.91
If you care about what the numbers mean you can read it in the opening post in the five-speed vs. six-speed thread: http://www.advrider.com/forums/showthread.php?t=281742
Here are some threads:
fuel mileage 225 vs. 250 http://www.advrider.com/forums/showthread.php?t=319350
on the 250 http://www.advrider.com/forums/showthread.php?t=290853
Other ADVriders you might want to PM for information are justacommuter and The Cameraman.