My 2003 Avalanche has the dual manual controls and my 2004 EXT has the dual auto controls.
Each system has it's own benefits and shortcomings.
While the auto control does allow for a more set and forget style of operation, meaning I tend to set one comfortable temperature and not change it very often, the rotating temp knobs do seem to have problems where the temp selection does not adjust as it should until the knob shaft is cleaned with electrical contact cleaner.
The auto selection of the fan speed is nice, as long as the fan speed fits your needs at the time.
About the only time the auto system goes full on fan speed is when you first get into a smoking hot truck cab in the blazing heat of a summer day.
And then for a fairly short period of time.
I think GM was more concerned with quiet operation than anything else.
With the auto system, one common failure point is the small ambient air temperature sensor in the headliner above the driver's head.
This device can make a buzzing sound as it's small fan begins to fail and is often mistaken for some sort of speaker, which it is not.
In my Avalanche with the dual manual controls, I find myself reaching for and adjusting the fan speed more than I do in the EXT.
Not constantly, but more when I think greater airflow would be needed.
And while I tend to leave the cold setting full on cold, I do have to use the manual slider controls to adjust for any warmer settings that may be desired at the moment.
During hot weather, I tend to prefer the recirculation setting to be activated almost all of the time.
Getting the auto system to select recirc is not as easy and it seems to not want to stay with that setting turned on, while the manual control will take the recirc setting and keep it until a different mode is selected.
During the heat of summer, I tend to prefer to set the a/c for full cold with recirc on and I will set my fan as desired for the moment.
Once again, I think GM is looking for a more quiet operation overall.
There are ways to force the auto system to do what I want, but then what is the point of an auto system?
When using the windshield defroster settings, the manual seems to be better when you desire full hot and high speed fan operation.
Selecting defrost on the auto system seems to always select a much lower fan speed than what I would like to get a lot of heated air on to the windshield as quickly as possible.
The auto system does not seem to auto detect when a good defrosting is needed.
The auto system is more for auto selecting cabin heating and cooling.
And if I remember correctly, when using the auto system, the floor heat and defroster do not even allow for higher fan settings before the engine cooling system reaches a certain temperature.
I found this a little distracting when leaving the house on a cold morning and the heater would not begin to operate at all for a couple of blocks.
For this reason, on really cold mornings, I would tend to let the EXT warm up for a few minutes before heading out.
The engine did not have to be full on warmed up, but still.
And to be completely honest, If I could only have one system, I would lean toward the manual over the auto just because I sometimes have to defeat the auto settings to get the operation I desire.
The auto system is different, but not perfect.
The manual system just takes your desired setting for the moment and does what you ask of it.
With that being said, I do not see where it would be worth the time, trouble and expense to switch from manual to auto.
