They're different steppings / revisions. U-0 is more recent than M-0.
The first version of a new microprocessor product is the A-0 core step. As Intel makes functional fixes or manufacturing improvements, the core stepping letter and / or number is incremented. Minor changes result in an increased number (for example, A-3 to A-4), while more complex changes result in the letter being changed (for example, A-3 to B-0). B-0 core stepping is more recent than an A-4 core stepping.