WordPress Multisite, by default, does not support using multimple domains. But you can easily add mult-domain support with a domain mapping plugin.
So first get Multisite running with either sub-domain (wmzfm.wtixfm.com, xyzfm.wtixfm.com, etc) or sub-directory (example.com/wmzfm, example.com/xyzfm, etc) and test to be sure everything is working.
Next, get a domain mapping plugin to map the individual subdomains (or sub-directories) to their respective domains:
wmzfm.wtixfm.com => wmzfm.com
xyzfm.example.com => xyzfm.com
Etc.
There are several of these domain mapping plugins in the official WordPress domains repository, but my favorite is WordPress MU Domain Mapping: https://wordpress.org/plugins/wordpress-mu-domain-mapping/
In your opinion, which would be easier or better to do?
“Easier” is definitely subdirectory, as there’s no additional step of DNS configuration.
But “better”?
In your particular use case — 100% custom domains — I don’t think it makes any difference at all.
If properly configured, neither users nor Google will ever see the sub-domain or sub-directory URL. So there’s no impact on SEO or user experience with either option.
And I don’t know of any performance difference between the two.
So, in my opinion, in your particular use case, it just boils down to personal preference as far as “better” is concerned.
(Oh, and by the way, I just discovered that domain mapping is now a native feature, so no plugin needed at all. https://codex.wordpress.org/WordPress_Multisite_Domain_Mapping)