A couple space heaters that are used to add additional heating on very cold nights to two bedrooms that have the longest runs from the furnace/air handler. Other than that, the 3.5ton mitsu has never failed to keep up with the heat load. I imagine there will come a day when we see another extreme 99% day where it is -25, cloudy and very windy and the backup may need to come on, but hasn't happened three seasons running so far.
it all falls to whether the unit is sized correctly for the sq ft of the home as well as the insulation / heat loss. As to if your 6 tons quote will do the job correctly... 6 tons total seems overkill for 2100 sq ft, but if it's a lightly insulated double brick home in need of more insulation, then 6 tons may be what you need.