As a current owner of the Spektrum DX6i, as well as the Spektrum DX8, I can tell you that there is one difference that I can speak about. That one difference being the smoothness of the sticks on the DX8. The sticks on the DX6i seemed to more difficult to move. Don't get me wrong, I was able to fly my planes and my Trex 500 with the DX6i, but once I flew my planes and my 500 with my DX8...I noticed an immediate difference with the sticks. The sticks on the DX8 move much smoother.
Now, the DX6i did everything that I wanted it to do and I upgraded to the DX8 for the extra channels and the 3 position switch for Flight Mode. As far as the settings from the DX8 to the DX6i...I can't imagine there will be many differences in the setup but I'm sure you will have to tweak your settings a bit.
Good luck!!!