Rally is the best stamina skill in the game...or sometimes feels that way when you don't have major mending as a NB and hit it and you get full health back. So you are bearing through levelling 2h? But do not plan on using it? Bow as secondary would be great so you get around faster/kite range options(utility is still the best). As a main weapon for raw damage you'll want either 2h(less damage than DW but more survivability, use a greatsword) or DW(Highest sustained damage possible and will make solo content easier as long as you have vigor, use two daggers or two swords). If you use 2h you'll need to use power extraction for some AoE game, DW gives access to a AoE in steel tornado. If you are using Aoe that is.

For pvp on a nightblade SOLO* bow and 2h forever. I only go DW and 2h in a group because the only contribution us NB's have is high burst+defile on tanky guys AKA nothing.

EDIT: I almost forgot, when I started the game I used bow and dual wield like how I did in skyrim, 2h felt terrible to use. Once I tried it out for rally in pvp I never take it out of my builds...its a beautiful love story really.