Depends on how you are using the team.
I strongly believe zAV is superior in defense, while zMT is superior on offense. In either case, NSS hits like a FREIGHT TRAIN at higher relic levels (she’s an Agility Attacker, they gain the BEST mastery stats in the game), and you won’t regret bringing her there.
NSS is much more menacing under zAV than zMT. If you use NS on defense, I suggest you zeta AV and bring NSS to r5 ASAP. If using your NS for offense, I’d suggest bringing MT to r5 and stacking her with as much offense from mods as possible... I’ve seen her AoE one-shot many toons at high relic levels (albeit that was under zAV with offensive boost from her leadership)