For art for its own sake: They're not bad.
For art for a cover: Not my cup of tea. Though to answer the question, I like the Tiamat one best for some reason.
I'm not a huge fan of random poses. I like context and story. I also like a bit of realistic looking (don't care how practical) art. I enjoy a well designed alley in a big city with nice bits of bobs to look at and random people in the street doing things. I care less about a spread of every character just facing the "camera" or the more recent "landscapes" I saw coming out of WotC - with the one singular base in the middle of the craggy landscape. I'm sure it looks cool, but no one would actually build something like that outside of a fictional world because it is too impractical to get to the front door. And that second part ruins a lot of atmosphere for me. So, yeah, Tiamat's base is shiny.. but why does it look that way? What about that screams, "Tiamat the destroyer, fear me!" because to me, nothing does. The same kind of goes for these covers, they're detailed but they lack context. They're flare but without substance. Like comparing the iconic Fonze to some random dude wearing a leather jacket with a popped collar. One thing is actually cool and one that just thinks it looks it. But a hard thing to define, except to say one thing isn't the Fonze (Ehhhh!).