I have already said multiple times why I support
Ground.
To those who use weaknesses as reasoning for Poision, a pokemon for you, Celebi. Celebi has 7 weaknesses, Ice, Dark, Ghost, Poison, Bug, Fire and Flying, of those, 5 see common OU use, and one of them is a x4 weakness. Celebi also happens to be decent at defending against the Secondary, because of Natural Cure, although it dosen't STOP the secondary itself.
Dark/Poison gives us a Ground weakness, and Drapion is already a pokemon with the same type, decent movepool and stat layout. It dosen't break into OU, it's not even common in UU. Dark/Poison would be walled by the myriad of Steel types, and those carrying Earthquake/Land Power would happily kill it.
Also, Toxic dosen't stop a sweeper doing it's job, it just limits the turn count. Paralysis, however, does stop a sweeper, and makes it dead weight from the instant it happens. If CAP9 can punish Trick and Paralysis, it benifits both walls and sweepers alike.
My vote is for
Ground.
@ Flareblitz: Those stats are from OU. The CAP metagame is different. Stealth Rock should count as well, seeing as you are counting T-Spikes for Dark/Poison, and Ground/Dark resists them. And, seeing as the highest for Dark/Poison is not even 10%, it's a ton easier predicting the Thunderbolts, which are an immunity, than the Surfs, which will still hurt, especially from Starmie, and Emploeon, the latter is suicide to switch into for CAP9, either way, although Ground can revenge it. Count all Hippowdon and Tyranitar too, please.
Also, I took the libery of Fixing FamilyGuyMan's chart:
The resistances are huge factor. Firstly, I don't know why are you are switching into something that commonly uses Earthquake with a Dark/Poison Pokemon. How will Dark/Ground switch into U-Turns? Surfs? Leaf Storms? Ice Beams? It may not take full damage from SR or SS but here's some stuff that could be interesting:
Dark/Ground's Advantages Defensively (What Dark/Ground takes better than Dark/Poision)
Earthquake 21.17%
Tyranitar: 16.59 % (Sand Stream)
Thunderbolt 13.82%
Stealth Rock 11.7%
Stone Edge 10.3%
Hippowdon: 4.56% (Sand Stream again)
Thunder Wave 4.04%
Earth Power 3.56%
Thunderpunch 3.08%
HP-Electric 1.47%
Total: 90.29 %
Dark/Poison's Advantages Defensively (What Dark/Poision takes better than Dark/Ground only)
Surf 9.74%
Ice Beam 8.73%
U-turn 7.91%
Close Combat 6.14%
Waterfall 5.92%
Ice Punch 4.88%
Superpower 4.43%
Toxic 3.6%
Grass Knot 3.57%
HP-Ice 3.4%
Brick Break 2.79%
Focus Blast 2.39%
Hydro Pump 2.04%
Ice Shard 1.95%
HP-Grass 1.9%
X-Scissor 1.74%
Ice Fang 1.66%
Focus Punch 1.34%
Dynamicpunch 1.24%
Leaf Storm 1.2%
Seed Bomb 1.02%
Energy Ball 1.01%
Total: 78.6%
Ground wins, by a margin greater than Stealth Rock's useage.
Then, start counting OFFENSIVE advantages. Ground wins 100% there. Breloom and Celebi vs Metagross, Jirachi, Tyranitar, Heatran, Lucario, ect.
Oh, and, move-wise, there's 400%, 4 moves per pokemon, and it's times moves are on a pokemon.