Well to be completely honest, to be competitive you need PvE pieces of gear. You could donate, do PvE or sell chars etc. to buy the PvE pieces you need. Fortunately, DK's don't need a ton of PvE gear - weapon and trinkets are enough. Rolling human is a clear advantage though, because you can equip 2x trinkets after all.
Frost is the better spec for 2vs2, while unholy is the better one for 3vs3, but only if you have good teammates. Thats the real issue imo, given how much warmane players avoid 3vs3 for whatever reason.
The pros of unholy are your pet, magic damage resistance talents and better survival due to bone shield. The cons of unholy is the lack of burst and the fact that your main CD Gargoyle can be killed/ccd/los, while frost has huge burst capability and great CC due to Hungering Cold, but lacks the survivability of unholy.
Imo both specs arent that hard to master, but frost is the easier to play since all you need is to grip a healer and HC him - that would force a cd out of him, or would guarantee you a kill, while unholy requiers coordination with your healer and games take a long long time (talking about 2vs2 here).
In 3vs3 as unholy youre mainly the peel/support dude, so it works better for most viable comps.
You could switch easily between specs, since most of your gear would stay the same, except for weapons.