Thief is the most efficient mundus stone, both for pve and pvp. There's a case to be made for shadow on builds that are around 50% crit without thief on pvp builds. Even with capped crit resist, which very few players have, you're only negating base crit damage bonus. Basically, everyone wearing impen is irrelevant because crit is that strong.

I would advise against running either atro or serpent mundus, they're both equivalent to 1.75 set bonuses, whereas the thief is worth almost 4 set bonuses. It is much more efficient to build necessary regen into your build and seek your crit chance from the thief than it is to build crit into your build and get regen from atro/serpent.

If you're currently having success with shadow there's no reason you won't continue to, you may just need to drop 1 proc set for a more traditional damage set if you're stacking 2+

By the same token, I wouldn't run warrior/apprentice becuase they're only ~1.3 set bonuses each, there's better ways to get your weapon/spell damage.

TLDR: Run either thief or shadow for pvp