Mullard 12AX7 10M gold pins vs. steel pins

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 European versions of the 12AX7(ECC803/ECC803S/E83CC) usually had the gold-plated pins, thicker or triple mica, matched triodes and were better built for longer life/lower noise(the "S" indicated "select" and they were especially quiet and balanced). You would hear an improvement with these tubes, but- if you enjoy the added warmth that the Mullards provide, you may miss that with some of them. The 10M Mullard is built to the same standards(and even more rugged), will provide a platter-full of improvements, and still possess the sound you're enjoying now(well- maybe a bit less microphonic).