With dual rates... you are limiting the total amount of servo travel that your transmitter stick can produce on the model. The amount of travel is linearly increasing consistently amount across the full movement of your stick. It's probably a little easier to understand when seen on an airplane.... Say you have 2 rates on your elevator, one at full and one at 50%. Lets say at 100% rate your elevator moves 3" when your stick is moved to it's fullest extent... at 50% it would only move 1.5" when the stick is moved to it's fullest extent. The amount of movement during the first part of the stick movement is consistent and the same as the last part. In otherwords... at the 100% rate if you move your stick 1/3 of the way of it's full travel... your elevator would move about 1" ( in the example given ) and each additional 1/3 movement the elevator would move another 1"
Here is a graph of a 100% dual rate showing a straight linear progression of stick movement vs the control surface movement.... ( ignore the ghosted out background levels that are shown, they aren't relevant here )
Here is a 50% dual rate showing that the full movement of the stick only results in a 50% movement of the control surface
With exponential... the amount of movement of your transmitter stick produces a movement at the control surface that increases or decreases exponentially across it's movement range.
An approximate 50% expo set to "soften" ( decrease ) the control surfaces movement during the first part of the sticks travel vs the later part where it increases until it reaches the full amount of movement for the rate that is selected.
Here is the reverse of that... where the stick is more sensitive during the first part of travel producing more control surface movement than it does during the last portion of stick movement.
One final bit of info.... You can use both D/R and Expo together if you wish.
As to which is best for someone... there isn't a single answer that will fit everyone. It's subjective and will be unique for each person. I prefer a D/R over an Expo because I judge how much stick I am using based on the response the model has made. I move the stick until the model responds the way I want it to respond but.... I fly planes and not heli's. If for some reason I don't get as quick of a response as I'd like, I increase the rate. If I want to fly aggressively... I'm going to be using full rates and if I want to just take things easily... I'll switch on a lower rate ( I typically setup 3 rates on my models ). Typically while I'm flying and making maneuvers, I'll have on my high rates... when landing I switch over to my medium or low rate.
