DMSS for the reasons below.

I have both as well as some leftover stuff on XPS. I have moved all my expensive airframes over to DMSS (XG11). It not only feels better in the air, but if you look at wavelength analysis and how DMSS uses the spectrum, it is as robust a FHSS protocol as I can imagine, starting with the capture bandwidth being 2.8MHz versus 1.1MHz for DSMX, the next most robust. The DMSS radios are so dominant on the spectrum that I wonder if it would pose a risk to weaker protocols like DSM at the field.

DSMX is also a solid protocol and is much improved over DSM, but DMSS is better. The other reason to go DMSS is the quality of the radios. The JR stuff is just better made and there have been few reported problems with JR radios against a lot of issues for Spektrum (even though the Spektrum stuff is supposedly made by JR).

The downside to DMSS is that is cannot be used with any of the DSM/DSMX stuff. I have a DSMX radio for my cheaper airframes (DX6).

Programming and function-wise, the XG radios are almost the same as a 12x, but menus are a bit more intuitive. Coming from a 12x, the XG interface will look instantly familiar.