Well, I really like the variable speed fan on my unit. The bigger thing is you really need an analysis done on the house to determine the proper sized unit. A two stage option can be nice. For best comfort, you want long runs. Since the weather isn't below zero every day, having a variable output can help. A variable speed fan prevents the cold surge and noise you get when the thing first comes on since it ramps up, and at the end, ramps down. You may not notice that it's running unless it ever (may not happen often) ramps up to max speed.
You can get a fair idea of your actual, as used, heat load if you take the amount of gas used and compare that with the degree-days for that month(s). A therm is about 100K BTU (you can get more exact if you wish). See how many therms used, then how many heating degree days there are in the period, and you get the BTU/day, then divide by 24 for the per hour. Depending on what else is run by gas in the house, if you subtract say a summer month's usage from a winter month's, you should eliminate things like a gas dryer, stove, or WH from the equation. It wouldn't be uncommon to have a real heat load in the order of 30-40K BTU. A two stage unit an meet that often on low-fire. Longer runs means better efficiency and less wear and tear on the equipment and comfort is improved.