SABLEYE:
Sableye gets the ability Prankster so he has Priority Will-O-Wisp, Toxic, Substitute, Recover etc, you could also add things like confuse ray, night shade, protect, swagger, making him a great staller/crippler.
So now you can invest in defenses and HP, and with the opponent unable to hit super effective, he's tough taking down.
UMBREON:
Nice amount of HP, and incredible defenses.
Umbreon gets moonlight for healing, and you can add toxic protect here to make him a staller.
curse also helps, boosting your defense/attack even higher.
Umbreon is more of the staller while Sableye is more of the annoyer.
I would go with sableye, as he is not weak to bug as your other dark types should be, at least some of them, and he is imnune to fighting attacks, so if you see a fighting type threatning your other dark types, you can go into sableye.
