What each one actually does
A drain and fill empties the pan and refills it. Much of the fluid stays in the torque converter and cooler lines, so you replace part of it — repeating it a few thousand miles later gets you most of the way there gradually. A flush connects a machine to the cooler lines and exchanges nearly everything in one go.
The honest position on flushing neglected transmissions
There is genuine debate here and we will not pretend otherwise. The concern is that a transmission with a lot of miles on old fluid has worn in a particular way, and a full exchange changes its behaviour suddenly. Some technicians consider that risk overstated. We tend toward the conservative option and will explain our reasoning for your specific vehicle rather than applying one rule to everything.
If it is already slipping, neither will fix it
Fresh fluid does not repair worn clutch material. If the transmission is slipping under load, flaring between gears, or shifting harshly, that needs diagnosis. Anyone selling you a fluid service as the cure for a slipping transmission is selling you something.
Fluid specification is not negotiable
Modern transmissions call for fluid meeting an exact manufacturer standard, and the wrong fluid causes shudder, poor shifting or damage. Universal fluids that claim to cover everything are a false economy.