I think the argument against variable speed is probably wrong but....

If this opinion is based on a thorough inspection of your ductwork, then it might be right. Obviously lots of us have variable speed blowers and don't have an issue (in much larger houses). Generally, a lower fan speed leads to more even heat distribution.

Do you have dampers? Do you have decent insulation?

In my new house, I have registers at the walls - which is a pretty old school idea. The idea is to heat at the walls since that is the coldest part of the house. Well - the reality is that it doesn't matter much on a new house. But in a/c season, it does probably help to be near windows since solar gain is what you are fighting.

It doesn't take much airflow to get heat away from the furnace...

You may be misunderstanding the terminology. The term "variable speed" refers to the blower in the furnace. So, it is possible to have a single STAGE furnace, with variable speed blower. Your typical (older) furnace may have a multi-speed blower. where the installer 'selects' the required (single) speed through electrical taps.

Further, in the above case, the speed of the blower is not variable (i.e., you don't have a choice of fan speed, while heating. As with any gas furnace, the fan speed must be selected, such that the air flow through the furnace will produce a temperature difference (known as temperature rise) of the return air to the outgoing air, within specified limits. What a VS blower WILL do is ramp up in speed if restrictions (clogging filter, poor duct system)prevent the selected airflow from flowing. It will (with proper thermostat) reduce airflow in cooling to increase dehumidification.

A 2-stage furnace (also available with VS blower)and proper thermostatic control, provides greater comfort from having longer running cycles. NOTE: A 2-stage unit provides greater comfort, not greater efficiency.

Carrier/Bryant produce a 3-stage furnace, which they refer to as 'modulating', but there are larger gaps in output between stages.

A modulating furnace provides many stages of heat and utilize variable speed blowers. Again, the ultimate in comfort, but not more efficient than a single stage unit of the same AFUE.

You should note that newer furnaces blow a lot more air than older units of the same size. Combine that with a VS blower, if the duct system is not sized for the airflow, the system will be noisy and efficiency may suffer. So, proper sizing and duct system analysis is essential.

On a personal note, I replaced my oversized SS furnace with a Rheem Modulating unit (13 stages). Before changeout, the upstairs was always cold; the downstairs would warm, but would chill before the unit would restart. I had to use a space heater in an upstairs bedroom (computer). Now, I have relegated the heater to the basement, and frequently sit at my computer in short sleeve shirts.

Insist on a manual J (heat calc) and duct analysis.

V