I have them both.

I love my DKs
I love my paladins.

They both offer a unique perspective, and a different strategy. DKs with glyphed rune tap have a massive amount of healing and survivability. 4-5 Unholy with Rune Tap have excellent AoE and survivability.

4-5 Rets are a powerhouse in PvP and can easily with a good tank solo almost any heroic in the game. Great power, good healing (though, not as nice as the DK's for raw aoe healing at your fingertips)

If I were to choose one, I would choose my Paladins. But, a ret/prot and 4 DKs are amazing in PvP too.