Each company's track records speak for themselves.
SoftBank pulled a struggling carrier from the brink of bankruptcy to a very competitive third place challenger that now has enough money to buy a controlling stake in an established, major US carrier. They have clear goals and a plethora of possible new business directions.
Dish, on the other hand, is closing a majority of Blockbuster locations, while struggling to establish Blockbuster (who used to be the de facto movie rental business) against Netflix and even RedBox, who themselves are struggling to compete with Netflix. They are hemorrhaging money as they try to find ways to compete against cable companies, AT&T, and even DirecTV in an era of IPTV, The Pirate Bay, and Google/Apple TV.
As has been pointed out, Dish has no clear direction here, and no idea how to compete in a changing market. And considering how drastically the current mobile device landscape is changing, that's a huge disadvantage. SoftBank is definitely, clearly the best choice in the long term. Unfortunately, once the shareholders see those dollar signs, it's hard to ignore them, and it may prove ultimately too tempting. If Dish can pump enough money into this deal, I'm sure they can persuade a substantial share of the investors. After all, aren't they investing precisely so they can make huge profits off of deals like this? And we know how shortsighted folks can get with dead presidents waving in front of their eyes. The long term becomes irrelevant.
Frankly, I'm hoping the shareholders make the right choice, but I'm prepared to leave if Dish wins. T-Mobile may not be perfect, but it isn't Dish.