All stam builds are pretty much DPS in PvP, regardless of class. I guess the templar would have more healing utility with cleansing ritual but you won't be able to be the main group healer, if that's what you're asking.
If you mean for a brand new player, leveling and such, the templar will be easier due to easier access to heals. However, PvE leveling is so easy these days that you shouldn't have any trouble with either.
If you mean in PvP, the NB would be harder simply because it is harder to master all the tools at his disposal (like the shade teleport). Templar is more straightforward, stand your ground and fight - however when you get in trouble, you will probably die where the NB might be able to get away.
Templars heal the best but they are stuck in the fight till someone dies. nightblades can easily cloak and leave a fight at will. Soo if they feel like they are losing a fight cloak, hide for a minute then choose weather to restart the fight on their terms or just walk away.
Wear 5 medium armor, 1 heavy (chest piece), 1 light (belt). That way you Maximize your armors resistance. Wearing 5 medium armor will eventually allow you to unlock a medium armor passive which increases your DPS significantly.
Make sure to get mass hysteria for your magicka dump.
Most nightblades dual weild because of bonuses for wearing sets of armors.
Stamplar has a more complete kit, with access to a class based purge and major mending. However, Stamblade has the ability to go all in on damage because of siphoning attacks and access to minor berserk and cloak