As per request, I'm answering this question a few days late.
Punishment or Foul Play, that is the question.
Well, Punishment is virtualy never used competitively, as it requires the opposition to boost before it can be of any use, and the more they boost the more dangerous they are, and your Punisher will most likely wind up dead. Not to mention that Dark is resisted by several common setup sweepers, such as Mega Gyarados.
Foul Play, on the other hand, it the saving grace of some defensive Pokémon like Mandibuzz and Amoongus. These two have pitiful offensive presence, and Foul Play lets them use the high Attack stats of the Pokémon they wall against them, and keeps them from being total Taunt-bait.
Foul Play is often used in tandem with Swagger, to form a move combo knows as SwagPlay, which is common on Prankster users like Klefki. Sableye has been known to run a SwagPlay set itself, and is successful to some extent. It does not, however, typically run Punishment (nor does anything else).
TL;DR Foul Play > Punishment
Here is a set for a SwagPlay Sableye (WARNING: If you are paying on Smogon tiers, then the move Swagger is banned):
EVs: 252 HP / 232 SDef / 24 Def
- Foul Play
If you are planning on using SwagPlay on Mega Sableye, then don't. Mega Sableye's most used and most annoying and stally set is Shadow Ball / Will-o-Wisp / Recover / Calm Mind. While it can potentially run a SwagPlay build, this is far inferior to simply being able to shut down Physical attackers with a Burn.