I have both Noor and Lady Loki maxed. Use them daily for several reasons.
Loki is definitely useful and she is in my 3-2 red team always. It’s great to cast all ailments back and do damage.
Noor, she is very useful in fast wars, map and ninja tower. In raids and non-fast wars, Noor fires when the match is already decided, and only rarely she is the reason why you win a raid. Used her a lot to fight minion teams, I still find her too slow to play a relevant role, and that’s why I maxed a 2nd Marjana, which I find way more useful thanks to fast speed + sniping skills.
Overall, I do not regret maxing Noor as she makes playing maps and quests easier, but she can’t be used in 2600+ raids where fast AoE heroes fires 3 minutes before Noor is charged, unless you find a very friendly board.
To summarize, Lady Loki over Noor for sure.