WoW Rogue. When a rogue goes invisible, he or she can do it so indefinintely, no question. Nightblade, however, can also become invisible, but only for a short amount of time.
Think about it, in order to safely navigate through a pvp zone, nightblades have to use the invisibility ability as intelligently as they can.
Rogues, on the other hand, just need to toggle it on, and as long as they don't get close to the enemy, they can be completely safe roaming the battlefield.
TL,DR: Rogue "Sealth" active ability > Nightblade "Invisibility" active ability. Anytime.
TL,DR: Rogue "Sealth" active ability > Nightblade "Invisibility" active ability. Anytime.