Though my favorite to fill the position is Lang, Mangini is more than capable. The good thing is that whether it's Mangini or Lang or even Minnemann DT will raise the bar drumming wise, and that can only be a good thing. To me drum solos are not the ideal place to evaluate the player's creativity as most of the time they are just showing off their technical skills.
Most of what I've heard from Mangini (The 3 LaBrie albums, Extreme (Waiting for the Punchline) and Annihilator) he plays really solid, and tastefully, yet in my opinion nothing to write home about, to me honest if it were for those recorded performances alone I would've never thought of him as a candidate. Granted none of those recordings feature music as complex as DT's. I'm sure he can pull it off and then some.
What I've heard from Lang, is mostly he's instrumental album StOrk, the music has a lot in common with DT in terms of progressiveness and heaviness, his performance is very creative and simply mind blowing, I guess that is why I tend to side with Lang.
When all is said and done, we will end up with DT with an incredible drummer plus the benefil of having discovered lots of new music with increadible drumming along the way, I know I have.