I fly with the transmitter on my hip for CL scale with large models. I have used the Bill Young handle that has the throttle trigger like the old 3-line handle. but is also had toggle switches for flaps and other features. When ever I reached up to flip the toggle I would jiggle the handle which would affect the level flight of the model. And because you working the trigger with one finger you only have 3 fingers to grip the handle. Then I started flying with the transmitter on the hip, this meant that one hand had to handle the pull, but you can get a better grip on the handle because you can use all 4 fingers.
The 13 lb B-29 that I fly pulls really hard (40 - 50 lbs) and I can still hang on. I find that I get a smoother flight when the flying portion and the throttle control portion are separated.
The other thing I don't like about the trigger built into the handle is that you can't adjust the throttle unless you bring the handle back to the model. With the transmitter I can just bring it with me, set the throttle and then walk out to the handle with the transmitter.
Also the newer transmitters have model memory which means you can set the end point adjustment and other settings it remembers the settings for each model. I fly 5 models with one transmitter, they just use a different model memory. The only thing I need for a new model is a receiver and set of lines.
I also have a set of lines and handle for each model, this means I set the neutral for the elevator and never have to adjust when I get to the field.
Also the RC car transmitters sometimes do not have the end point adjustment which I really need for electric power.
But in the end use what you like best!
Fred