From an overclocking standpoint, you want to pay attention mainly to # and quality of VRM phases. The gaming 5 has eleven and the gaming 3 has seven. So theoretically, you should be able to attain a more stable overclock on the gaming 5.
If you are happy with say, 4.8GHz - 5.0GHz then the gaming 3 will have a good chance of obtaining that (there are other factors of course). If you want a better chance of higher, stable overclocks go with the gaming 5.