Go for somewhere smaller, that is currently getting plenty of airplanes (hopefully positive growth when it comes to total fleet number), has a fast upgrade, isn't stagnant, and with bases you are okay living in. Since you want the fast upgrade you might end up on reserve as a captain for a while if things slow down at that point, or you might end up holding a decent line stuck as an FO for a while. Just because a place has a fast upgrade now doesn't mean something won't happen to stop that.
The only good choice you can make is based on the facts now. Rumors are just that and this industry is full of them. Things can and will change and there is no telling where you will end up, but hopefully you can get on somewhere that has lots of movement. That means people are generally pretty happy and moral is higher. Once it's gets stagnant it gets negative quick.