Well, I've been following this whole development (NFL in LA in general) for the past couple of years now. Los Angeles Stadium is an easier project to carry out and probably less harsh on the traffic environment of Downtown Los Angeles but Farmers Field is way further in development as far as pre-production and city recognition goes.
The benefits (new convention center, even more business for L.A Live, more residents and consequently more revenue in Downtown Los Angeles, twice as many jobs as Los Angeles Stadium would provide) of Farmers Field FAR outweigh whatever traffic concerns the city may have.
When we were thinking about building the Staples Center (and later L.A Live) in the late 90's people were worrying that it would congest Downtown Los Angeles, and it never did. Getting in and out of Downtown on the night of a Lakers game is easy. Plus, NFL games would be held on Sunday (sometimes Monday), the week's lowest traffic day.
Right now, as it stands, Farmers Field is the frontrunner in getting approved, the real estate team (AEG, who also built the Staples Center) behind them has just been making a better effort at getting the thing to actually happen.
The only way I see Farmers Field not being built is if the city doesn't approve of it because of environmental concerns, such as pollution concerns or if the blueprints violate some sort of code and is unfixable.