Hi i am going to make a new char for pvp its between the rogue and the death knight so which do you think? Thanks

Its really 2 completely different play styles. You cant compare the two but ill try and show you the differences.

Rogues:
Stealth/ambushing
Wear leather (Semi-Squishy)
High Burst
Decent Survivability (Recuperate)
*Not commonly used in guild raid groups.* (If you ever want to raid)

Death Knights:
Plate armor, not squishy.
Amazing Survivability
Plenty of self heals
Blood(tank spec) very viable for pvp (even in arenas)
Less damage than rogues, but the great survivability compensates somewhat for that.
Harder to kill healers/casters than rogues.

I personally would choose the DK, I always play defensively in any MMO, pve or pvp.

Rogues are the best at PvP. With more then 6 stuns and high burst damage you will easily wipe out all of the enemy even with fail gear. A do is MUCH harder in PvP but can't kill nearly as well