I like both, as they both have something interesting to offer (or at least their Megas did) and have really good typings.
If Megas are an option, I'd go with Mawile because it can hit like a truck and is super easy to train, while Sableye is mostly about stalling and annoying with RNG, which may be useful in some competitive play-styles, but for the main games Pokémon that hit hard and can defeat opponents quickly has allways been more useful (thanks to stupid full restores).
I had a Mawile on my OR playthrough, Sableye not yet, but it's a Pokémon I'd definitely like to give a chance some day.