The damage effect on the nightmare set is flawed as a design concept for knights.
The 5% reflection is worked out not from a base damage calculation but from the mitigated damage inflicted on you.
So when you succeed on shield defence the p.def of your shield spikes your base p.def and the damage reflected drops sharply, if you happen to have advanced block (where the p.def value of your shield is doubled for the mitigation effect of that attack) the damage is reduced to negligible levels. Kick in shield fortress and/or guardstance and the reflected damage is insignificant.
The advantage of NM lies in its high p.def, sleep/hold resists and p.atk bonus. Those are particularly handy for PvP, esp vs casters.
The 5% reflection, however is a bit of a cruel joke :( as far as its implementation on a p.def spiking class is concerned.
The Majestic set will likewise offer you a very solid build for anti melee PvP where the p.atk bonus and stun defence is important. The str bonus makes this the armour of choice for melee DD's. The lack of an integrated shield with an effect into this set is its limiting factor PvE and PvP alike.
DC is not a bad choice for SK's for PVE as its essentially a doom spin off. More con vs str trade off which the SK's tolerate well and the 4% heal bonus doesn't go astray. The 18% shield block gives a good defensive performance for PvE in conjunction with the Para resist. Lower P.def and no other specific resists do not make this the best of PvP armours.
You could also consider Tallum, the bleed and poison resists are solid for PvP, the extra portability gives good offset to carry extra quick heal and CP potions for PvP and other junk for PvE. Lower P.def and a lack of an integrated shield with a special effect are again a disadvantage for either PvP or PvE.
I hope this helps
Regards,
Ellyrion Fiallathandriel
7X Knight of Eva
Kain