Hi Derun93 -
1/gm = dVgs/dIds is inverse transconductance.
It always makes sense to distinguish between different resistances, such as differential resistance (Rd=dV/dI) vs "integral" (or total) resistance (R=V/I), as they have different significance, and often different values.
When a MOSFET operates as a switch, in most applications, ideally, it's resistance should be small when it is in ON state, i.e. its Vgs should be maximum, and it should operate in the linear region (i.e. small Vds).
Other than small total resistance (Rdson), power transistor should satisfy a number of other requirements - such as uniformity of current distribution over device area, current densities in interconnects (metal and via layers) within spec, etc.
Max