Both can be useful, but T-slots don’t have many vendors or accessories available. The 16mm holes are industry standard, so those will offer an infinitely larger pool of tooling to use with your table. And there are threaded hole adapters that will allow you to convert a standard hole to a threaded hole.
Once you’ve inserted the threaded hole adapter, it can be very useful to bolt projects directly to the table, and doing so will hold them far tighter than any clamp ever could.
So I would almost never recommend tapping holes on the surface of the table because then you definitely can’t use a standard tool in that location. Plus, eventually the threads will be damaged. So better off to just use a cheap, mobile adapter.