With a 40k low stage the 2 stage will not be a reduction in comfort or gas efficiency over a single stage furnace.
Single stage furnace with ecm motor.
Output of the motor can be adjusted by signals from a controller or permanently programmed to advance through a range of speeds during each on cycle of the furnace.
Keeprite bryant and carrier offer single stage with ecm motors.
Be sure to not use the built in staging timer if you go that route.
When a furnace or air handler with an ecm is installed on a good duct coil and filtration system with static pressures within proper design parameters it can perform flawlessly.
Instead either use a.
A variable or multi speed furnace monitors the temperature of your home and then adjusts the speed of the blower motor sometimes referred to as an ecm motor in very small increments.
Variable speeds unlike an on off psc motor a furnace ecm motor can operate at a range of outputs between zero and 100 percent.
Is it worth getting it with a emc motor i do not have the fan on all year round and the furnace is only on 5 6 months for winter and on and off for 2 months of the summer.
And the ecm motor will have greater electrical efficiency.