Nobody knows enough about either one to determine what it's doing yet. The inventor of the Cannae drive hypothesized that it was exploiting some relativistic effect of the microwave reflection. One of the predictions he made, that certain slots were necessary for the effect, turned out not to be true. The inventor of the EM drive believes the Cannae drive is just an inefficient EM drive.
The device NASA built, which started out as a kind of Cannae drive but now does indeed seem to be an inefficient EM drive, is hypothesized, by the NASA group, to be accelerating virtual particles. The same device is reported to show some signs of possibly warping space, again by the NASA group.