Sorry-more info! Natural gas!

unfortunately I can't tell you off the top of my head what the costs are between the gas and electric. I guess I could try to hunt that down.

"The 16 Seer duel fuel has to be at least 3 ton and our units only need to be 2 ton each"

This doesn't make sense - please explain more. Are you saying that units less than 3 tons are less than 16 SEER? If that's the case, and is true with other brands too, I'd still opt for a properly sized unit and forgo the extra few SEER points. You'll save money and be more comfortable running a smaller size, no matter what the SEER difference.

The last thing you want to do is put in too much AC capacity - ie, too many tons in total. Especially where you live, removing humidity is essential for indoor comfort and to do that you want properly sized equipment that will have long run times. Or, multi stage units that will continue to run on low for humidity removal.

Be sure that a competent load calc is done with no number plugging and no rigging of the answer, otherwise the sizing is just guessing and you'll likely wind up with too much capacity.

Your HVAC guy is not giving you good advice. If you will have gas in the house then a furnace makes the most sense in terms of operating costs and comfort for most parts of the country. Equipment rated at 14 SEER in the South is typically low end builder's grade.

How large is this house? It must be very big in order to justify having 6 tons of cooling. Perhaps there are options to combine two systems into one with zoning.

the house is about 3900 square feet and kind of L/T-shaped. the main living areas are kind of in their own section, and then the bedrooms and bathrooms are stacked on top of each other like the top of the T.
we are now foaming a good portion of the house, so he has been reworking numbers and figuring out what we need. my husband had originally wanted a dual fuel system, but our guy let us know yesterday that that option would be out (due to foam) unless we go down to a 14 Seer. he suggested we should go with gas and stay at 16... and I wanted to confirm this was the right route!

Dual fuel made sense 10 years ago when natural gas prices were much higher than they are today. Call your utility company and ask what the prices are per therm for gas, and kilowatt hour for electricity. A simple calculation can be done with the numbers to see if there would be any savings with a dual fuel system.

You need a heating and cooling calculation regardless of what system you choose.