Never heard of an Airtronics M-4X. Do you mean MT-4 or MT-4S?

But that is a mid range radio, and although very good, it is not as fast and doesn't have the same physical adjustability features etc. as the high end model.

I have seen the 4PX in real and the finish is better than the M12. But the K1, to me, has the best physical quality finish and feel.

In that category, the M12 has a shamelessly cheap finish to it. I own an M12.

The MT-4 and MT-4S is even worse than the M12. Not saying they will break and won't last, just that the finish is "dissapointing".