To be clear, I can't predict who will win. My guess is the parties will settle with the Kirby's getting money, future profit participation, and assigning their right to develop their own properties based on the characters back to Marvel. But, while that's speculation, what's not is what the main issue is. As relayed in the NYT recently:

In many ways, the Marvel case is simple. It turns on whether Mr. Kirby was working as a hired hand or whether he was producing material on his own that he then sold to publishers. The Copyright Revision Act of 1976, which opened the door to termination attempts, bans termination for people who delivered work at the “instance and expense” of an employer.

Mr. Toberoff and Marvel disagree on the circumstances under which Mr. Kirby created or co-created the trove of characters.

....

If Mr. Toberoff has his way, the picture painted in court will be one of chaos. He says that during Marvel’s early days — when Mr. Kirby was creating his superheroes — the company was a shoestring operation that was barely afloat.

“There was no bullpen; there was a one-man office,” he said, contending that an industrywide decency code put so much pressure on Marvel that few at the company were worrying about contractual niceties with artists like Mr. Kirby that would have tidied up all of the legal issues surrounding work arrangements. “It’s easy to imagine that nobody at the time was thinking about work for hire.”