What are the differences one should consider between designing an RC snubber across a switch and across an inductor? Is there any preference for putting it across the switch?
A RC snubber can be placed over a switch, sure. Just remember that electricity is not instantaneous; it flows 1/100th the speed of light. For most practical purposes, it wouldn't make much difference if the snubber were at the switch or inductor... unless you're using very long wiring. Then transmission line effects come into play.
Also be sure to use a critically-damped RC snubber for best results.
A snubber diode may be sufficient across the inductor, provided it is fast enough and the rest of the circuit can tolerate a slight negative-going pulse and ramp-to-zero voltage for some milliseconds. If the inductor is a solenoid, it will have a slower release time.
Diode snubbers have the added benefit of wasting practically zero energy while the inductor is energized. (An RC snubber "charges" with the inductor, wasting some power each time it is activated.)