Looking at the individual WAR components Mitchell is at +9.1 batting, +5.2 base running, and +5.3 fielding runs so far for his career.
Over that 1997-2009 window looks like Cameron was at +6.2 batting, +3.9 base running, and +8.7 fielding runs per 557 PA.
Part of that base running edge for Mitchell also comes down to UBR (going 1st to 3rd, 2nd to home, etc without making outs on the bases) not being tracked for the first five years of Cameron's run so he's probably losing a couple two tree tents of a run there.