It depends on what you are hunting with your Culexus. If your opponent is decent they will make it so you do not have a favorable position to infiltrate your Culexus. It will be pretty obvious what the Culexus' target is too if your opponent has psykers.
From what I understand Sisters are used for area denial to help stop Psyker oriented death stars. You can take them in a single unit, but the Formation offers some slight buffs to them, especially if you play in a psyker heavy meta. They are useful in that they can be deployed forward as a psychic shield for your troops. They are also not half bad in combat as I believe they have AP2 weapons at initiative.
Their only drawback is that they are T3 with 3+. That won't last very long out in the open. Either way, they are certainly a good choice to keep melee oriented and psyker oriented "close combat" guys out of your gunline's hair.
TL;DR: If you see a lot of psykers in your meta, use the Culexus or Sisters and see which one works best for you. Culexus goes after the psyker. Sisters prevent them from getting to you.
Sky IS Falling, T'au WILL Suck, Sell Me Your Models