With both Hitachi and CAF already producing 125mph bi-mode trains, and TPX already having both Hitachi and CAF trains in their fleets, my guess is that they are more likely candidates than Stadler.
Another problem with the FLIRT is that the bi-mode model at the moment is only built in 3- and 4-car models, but with those carriages being smaller than other trains, that's equivalent to only 2- and 3-car anything else. TPX would need at least 6-car FLIRTs if that was the route they went down, but would the single diesel generator be sufficient for a bigger train? I'm not saying that it's an insurmountable problem, but it means that it wouldn't be an off-the-shelf design, so more expensive.