Sonically, unless the tubes themselves are actually different in construction i.e. the only difference is in the gold on the pins, initially, I don't think the average expert will be able to tell the difference between tubes steel pins and gold pins. (I've not heard these two Mullards however.)
The benefit in gold is that it doesn't corrode the way steel does and the pins don't need to be cleaned (as often).
The only down side I can see for gold pins is that typically they are nothing but gold plate over existing steel pins and what they will do, in some units, is expand the contacts in the tube sockets which might require re-adjustements/replacements if/when you go back to steel pins. Personally I don't go out of my way to find tubes with gold pins, everything else being equal.
FWIW
The benefit in gold is that it doesn't corrode the way steel does and the pins don't need to be cleaned (as often).
The only down side I can see for gold pins is that typically they are nothing but gold plate over existing steel pins and what they will do, in some units, is expand the contacts in the tube sockets which might require re-adjustements/replacements if/when you go back to steel pins. Personally I don't go out of my way to find tubes with gold pins, everything else being equal.
FWIW