Natural gas bbq and propane bbq should be comparable in terms of BTU. I seem to remember that my propane bbq has 4 x 17000 BTU/hr burners, 1 x 12000 BTU rotisserie burner and 1 x 12000 BTU side burner. Total cooking area is a little over 700 sq. in.
My house is gas ready (there is a gas pipe exteneded to backyard for BBQ), but I bought propane BBQ..Gas BBQ is expensive and you need to hire someone to install it for you (same as gas stove)..and I always think it is not safe enough expecially you don't have fence yet...besides, how many BBQs you do for a whole year yah...