Depends what you want to do. Arms warrior won't get a spot in rbg, frost DK might, they both can be flag carriers.
in arena? they are both fine people complain way too much about their own class instead of trying to improve their skills. if you don't have friend to play arena's with and intend to pug well arms and UNHOLY tends to be preferred by people.
also I think 2h > dual wield for frost DK in pvp. I'm no expert by far I'd suggest playing the one you like.
As others have said, Frost is basically a random BG spec where you play when you want to have fun while drunk and still put out decent damage and results. Its also probably the best Rated BG melee and target caller. Frost also does pretty well in low rated Arena. Contrary it popular belief, Frost's survivability isn't really that horrible. Its only really bad against cleave teams especially melee cleaves (basically a problem for all DK's)
The thing that sucks about Frost is that all it has is basically damage. Frost's toolkit is weak, so weak to the point that if you're not mindlessly tunneling, you're gimping yourself. It also feels like half a spec in pvp. Frost is well known to be a horrible 1v1 spec, you will feel completely outclassed against almost every spec in the game. If you do beat someone whether 1v1 or arena, it doesn't even feel rewarding at all, you just feel like you've gotten "lucky".
Arms is similar to Frost but definitely has a better toolkit. While Arms doesn't put out the same damage or slash damage as Frost, it does have better defensive capability against other melee making it a better choice in arena. It still suffers from the same issues as Frost, such as being a complete tunnel spec and feeling completely unrewarding when winning an encounter. However, Arms is better suited for people who would like to play a mindless melee with control and defensive capabilities.
If all else fails, just play Unholy. Its completely superior to both Arms and Frost in all aspects besides RBG's. 5.3 might change that though with Arms getting decent buffs and Unholy getting a massive indirect nerf to necrotic strike. The pvp power change also hurt Unholy much more when compared to Arms. So if you're planning ahead I would possibly go with Arms. Arms is also easy to learn and like frost, can dominate the lower brackets with ease.