I lived in Dallas for several years, and this was my observation, too. In some of the outlying counties around Dallas (e.g.-Hunt), "Coke" is definitely prominen

I lived in Dallas for several years, and this was my observation, too. In some of the outlying counties around Dallas (e.g.-Hunt), "Coke" is definitely prominent, but inside the urban sprawl that is DFW, "soda" is used first and foremost. I think it speaks to just how much of an influx there's been to DFW from the coasts in the past decade or so.