You never know we could get both eventually that would be nice
You do realise the reactor will new content too right?
We have the remains of the base level in game ATM but the reactor was essentially a huge floating structure like rata sum, we see a bit of it with the bonus event after fire ele but at that point most of it has already been destroyed.
I'm voting Kiel for two reasons, CoE is my fav dungeon in the game and I'm looking forward to a fractal of similar nature for the lore, mechanics and aesthetics.
Second, I honestly don't feel the Abaddon fractal can live up to the hype. It will be unlikely you will play as the gods taking him down (and if they did that the fight would be much less epic than it sounds, just look at the zhaitan fight). Most likely it will be playing as forgotten fighting a few margonites or something. I would love to see Orr in its original state but as the whole thing occurred in the crystal desert area that is unlikely.
If they had specifically said the fractal would be the sinking of the temple of abaddon then i'd be more interested. But if we had an orr fractal i'd much rather see the events leading to the sinking of orr instead.