Though this is moving away from the original question even though I mainly fly a fully 3D capable nitro .50 heli, I can survive with my Dx6i, and still even place a governor on it IF I use my head re what one I will buy. IE - even a model with 5 separate servos, a gyro and a head speed governor can run on a 6ch system if thought out correctly. Sure you lose some adjustment function re being able to turn the governor on of of by the TX, but I am not, not do I ever intend to be Curtis Youngblood or Alan Szabo re my flying ability, so I just don't need that level of TX options.
Secondly, I could not justify the expense of a DX7 after coming to the understanding that a 6ch system would so all I "needed". Also, I personally have yet to see a prop plane at the local club that "needs" anything more than a 6ch system, I cannot see myself flying a prop plane with more requirement than a 6ch system, so a 6ch system is what I have!!

As far as "travel adjust" function goes, while I'm not 100% sure, but i think this sets the absolute maximum travel on a servo re available throw on a given control surface (someone please correct me if I'm wrong), where the dual rate adjusts the percentage of that 'standard' off your switches (yet how they are really any different is beyond me - as they really just do the same thing from what I can see)