Yeah now that I think about it I haven't heard anything too bad about the s5 in terms of signal quality. My s4 is still hit or miss.Actually, they've made great strides with their radio quality since the S5 & Note 4 came out. I haven't come across any complaints in this department from anyone on our S5 sister forum. I think they've finally gotten their act together in this aspect.
That said.....all the metal on the S6....could be a problem.