Nimmo has the higher floor since he seems like he should be good for 3 WAR regardless and offensively won't hurt you as much. He's also three years younger.
Semien has higher upside if he can return to being a 4+ WAR player, which he's only one-year removed from. Two-years removed from a 6.5 WAR season. He's 35 now though which is a concern. Worst case, his floor is a 2-3 WAR second baseman over a full season as he proved in 2025, though offensively he could be awful if relying on him as a major bat.
Like the trade a lot overall from both perspectives. Me personally, I would take the chance on Semien since he gives you a solid glove at second base and if there is a possible dead cat bounce in him, he's capable of putting together a 5-6 WAR season.