in 2-5 weeks season 6 will start, and its a lot different. all teams start at 0 team rating and I think you always gain points, win or lose, until you get to some sort of stable rating. (not sure about the getting points on losing part but I vaguely recall that)

having said that keep in mind 5-boxing arenas is extremely, extremely difficult and I don't think there is anyone actually who has had any success in season 5 -- a handful (under 5?) of people had some moderate success in season 4, but never breaking 1900 rating afaik. Most people 4-box 4 DPS classes and have a second person be a healer.

DKs have gotten nerfed quite a bit, especially in PVP, in patch 3.1, they will still be very fun to play but I am not sure a full DK team will do that well in arenas. fwiw I played ~210 games on the arena tournament realm in 3v3 with 3 DKs, I went around 50-150 with a "single macro spam button" strat (a giant macro that had a castrandom of every single DK ability, lol) it was fun but there are some teams that just shut you down completely, I imagine its even worse in 5v5. I just played to get the murloc pet, wasn't really trying hard.

An all-dps cloth team like your second team is just going to get eaten alive by plate (plus rogues/hunters) teams, I don't see that one really having much success at all.

Either will be a lot of fun in battlegrounds, keep in mind as well Blizzard is probably going to do a lot more with battlegrounds in patch 3.2 and beyond - new BG, bg ranking systems, etc.