I think Haste/Vers is the go to for Balance in PvP which isn’t horrible for Guardian stats either. Little mastery mixed in for both is also a benefit.
Neither of them overly benefit from Crit so they match up fairly well.
And I’d say Necro would be the PvP go to covenant. That shield you can gain only adds to your tankiness as Guardian, and AS would play well with added pressure from Balance Affinity abilities