Using a 18 ch Rx does not give you 3 extra channels vs the CB 200.
One of the "extra" channels, 17 or 18 has to be used as an input from the sat Rx, in my case port 18.
I posted on the "other" forum how I can use port 17 for one of the rudders (it's a P-38) and I was told it's a 16 channel radio, not an 18 channel radio.
I was told when I bought it that it's a 16 function radio and several servo's can be combined to make up a function, but there seems to be no way to map port 17.
That being said, there doesn't seem to any reason not to use the CB 200 over the 18 ch Rx, other than cost.
Jon