By pleading to everything tossed at him Cohen blocked future prosecution risk.
I was under the impression he accepted a plea deal to avoid being prosecuted for more serious crimes. I did not follow his case closely so I am quite possibly or even probably wrong.
But the distinction here between Trump and Cohen with regards to the hush payments possibly violating a federal campaign finance law is that Cohen didn't fight it while Trump will. And of course that Cohen's case was federal while Trump's is not.