Adams is the much better player, there's no real question about that, and even if he became the hgihest paid safety in the league, you've got Mathieu on $14m, Earl just under that, let's say he cost $15m per year, he's a better signing than Clowney.
Clowney wasn't exceptional last year by any means, he had a couple of great games and that's it. I'd rather made a strong secondary into, by far, the best in the league than sign a high priced DE who barely gets any sacks.
Not only is Adams the better player, he gets more sacks too, so he could help out our DLine by playing that role, even more aggressively than he currently does. The defence would be better with Adams, the only area we'd really miss Clowney would be in the pass rush, but if you're Shanahan, would you rather face a Seahawks team with Clowney or Jamal Adams? It's not a difficult choice.