I currently use NGK XR 51X Iridium plugs in my 71 Road Runner. I fine they have worked well and perform great since installed 13 years ago. I only operate the car sporadically, so they have worked for me. Years back I installed some Champion plugs in a 440 challenger. One plug fouled out in about a month. Reinstalled the NGKs platinums that came out and never looked back. So in my opinion, the NGKs are superior.
Ron