The frame material also make a different and the Rush is steel so it will soak up a lot more buzz than the langster. If you really want a separate purpose-driven bike that is more utilitarian/off road friendly, then I'd wait for the Steamroller.

But really tire width and pressure will make the biggest difference here. You could spring for a new wheelset with wider rims, tires and lower pressure and it would make a bigger difference than the new frame for riding on the rail trail.