I vouch in support of both Poison and Ground. They're two great typing with their own advantages and disadvantages. Look:
Resistances:
Dark/Ground: Dark, Ghost, Poison, Rock
Dark/Poison: Dark, Ghost, Grass, Poison
The same amount, and about the same in usefulness.
Immunities:
Dark/Ground: Psychic, Electric
Dark/Poison: Psychic
Dark/Ground has arguably better immunities, allowing more switch-ins.
Weaknesses:
Dark/Ground: Bug, Fighting, Grass, Ice, Water
Dark/Poison: Ground
As you can see, Dark/Poison is better in terms of weaknesses, having only a 2x Ground weak. Dark/Ground has a whopping 5 weaknesses, all of which usually come together (Ice+Water, Bug+Fighting). Also, all of these are atleast common.
As you can see, they're the same in terms of defensive capabilities, however Dark/Poison is slightly (slightly=more than enough to warrant a notice) better. Now let's look at the offensive capabilities:
Types Hit Supereffective (with STAB):
Dark/Ground: Ghost, Psychic, Electric, Fire, Poison, Rock, Steel
Dark/Poison: Ghost, Psyhic, Grass
As you can see, Dark/Ground is miles better offensively, but most of the list is extra. Getting Ghost, Psychic and Steel helps the concept immensly (Ghost and Psychic more than Steel). Dark/Poison, albiet weaker offensively in general, hits the right types. Ghost, Psychic and Grass are the common types of common status users (Celebi, Rotom-A, Jirachi etc...).
All in all, both of these combinations have their disadvantages and advantages. It's up to you to choose which one is better.
EDIT: This may be in the wrong thread :S