Noblesse and Dreadlord are both incredibly fun classes. I find them both enjoyable to play as. I find it worth the time it takes to level them. Out of all the characters and classes in Elsword, I find these two classes the most worth playing.
Yet, if I had to say which one I favor and is better of the two. I would have to go with Noblesse. Certainly PVP is fun, but the best gear in the game for both PVP and Dungeons can only be found in Dungeons. Not only that, but Noblesse is still a deadly class in PVP. It can do a variety of attacks from long range that can hit anyone on the opposing team. However, Dreadlord does not have many AOE skills which are useful in Dungeons.
If you ask for my suggestion on which class to go with. I would say go for Noblesse because it can do well in both PVP and PVE. However, I would say it all depends on your play style. It really is a choice between getting up close and personal or annihilating things at a distance.