** existing electric cooktop can be replaced with a gas cooktop. We currently have a gas tank for our spa **
I assume this is propane. Most appliances have to be converted to use propane which can cause the burners to provide up to 20% *less* BTU's (heat) for cooking. Even with gas surface burners on modern gas appliances being in the area of 12,000 BTUs, many people are not satisfied with the heat they generate. This is especially true if they do lots of heavy cooking like large pots of pasta or canning.
My opinion is if you can find not a model that provides in the area of 15-16,000 BTU surface burners when running on natural gas, stick with electric (at least for a cooktop).