Necro is the better cov for resto, overall.
I get lower hps/dps burst, but much higher sustain and when looking at overall, it’s a net gain. That doesn’t mean other covenants aren’t viable though, I still run balance / NF on some keys or affix’s where melee doesn’t feel like the play. This week so far I’ve only done a 17 NW and a 16 PF, both wide open dungeons, easy to crowd melee even through quaking so I took feral / necro. My personal key right now is a SD, I’ll probably roll balance / NF for that one just because of the tighter hallways.
At the end of the day, it’s what you’re more comfortable healing with. NF gives you a burst healing cd and is incredibly forgiving, necro is a little more advanced because you generally want to run it with feral affinity and circle of life and death making it a more complicated playstyle, but IMO it’s very rewarding once you get the hang of it. Rdruid is one of the more open specs in the game, healing or otherwise. Feel free to experiment, see what works for you, and have fun.