I believe STV will offer Amtrak needed guidance in choosing the best bid. I hope that they are impartial to manufacturers, though Amtrak OIG is also watching this procurement very closely and should ensure that nobody plays favorites. Siemens, Alstom, CAF, Stadler, Hitachi, and Kawasaki are all great manufacturers who will offer Amtrak amazing value on these LD cars.
I still see Stadler as a long shot, just because they aren't as established in the US yet. I have seen their aggressive factory expansion plans which should be breaking ground shortly. With or without Amtrak, Stadler will be tripling the size of their facility in Salt Lake City. Of course I believe Alstom has the best chance since they have the facilities, people, and precedent. None of the other 5 have as much top-down experience with LD Bilevel cars.
I do think Hitachi and Kawasaki will be at a disadvantage because they mostly build metro trains and rarely branch out into other markets in the US. I do believe that CAF's Viewliner trouble will seriously harm their chances. Amtrak should be adverse to Siemens on this one because they don't want to monopolize their fleet.
I really do believe it will come down to Alstom and Stadler. And I'd love to see these cars come out of Salt Lake City, so I'm still rooting for Stadler.