Which one would fit better, Cole or Dorian?
For Jaws of Hakkon, Cassandra and Dorian are must-haves. There are important revelations about the Chantry that Cassandra really needs to be there for, and also a lot of findings about Tevinter that Dorian gives good commentary about. Solas is give or take, he has some vague lines, but I didn't miss him playing post-game.
For Descent, I'd actually take both Cole and Dorian, and leave Varric and Solas home. Solas has nothing interesting to say, and all Varric does is complain about how much he wants to leave (he actually does this in Jaws of Hakkon too, so he's not good company for either DLC). Dorian, on the other hand, has a lot of light-hearted but also insightful commentary, and Cole has really interesting lines about... some very important things in this DLC. Cassandra again gives a lot of good commentary in this DLC, so she's a good option too.