Both of them will slip the same approximate amount. The numbers come from real world observations, and 6% was based on knobbies on asphalt. Either way, the 426 is geared higher, and revs higher, and packs about 5-7 more horsepower. Both bikes in any kind of good health will top out well ahead of a quarter mile, so it will become s speed contest sooner or later.
In the SoCal deserts, I race against YZ's and other two-stroke 250's all the time (it's a mass start; all classes in each of two skill levels go off together), and all of them complain about the same thing anytime the course runs across a lake bed or a long section of high speed flats; "I could have kept up if I had more gearing/top end". Otherwise, it's a pretty even deal.