The difference is that the very context of Eureka makes NM different than dungeon. Like I said on another topic, in Eureka, you can enter from solo to a full group and freely change job or group composition depending on needs, and there's a bit of randomness in how you will roam in the zones depending on the NM that pops. You also don't necessarily partake in Eureka for the same purpose, depending of what reward you seek (Train, light farm, challenge log or the, saddly, few quests), and mostly, there is a way higher sense of community in Eureka, with people more willing to talk and help each other. Another thing is that, again, you already run these dungeons for something else. I'd take 240 dungeons and 100 NM over 400 dungeons, especially since, from 2.0, I've probably done several hundred dungeons already.