Building on what Daniel has already noted:

Indeed, judging quality of volreg or alignment with a single quantity is tough: if we had some quantity Q that could compare this well, we would use Q as the cost function itself!

Visualization is still *really really* important in neuroimaging---don't feel bad about that because it isn't a quantity.

To get a sense of what processing stream is dealing with motion "better" is a hard question. One criterion might be looking at correlation patterns across a group-- which are noisier, or smoothed out, or some other feature, likely due to poor(er) alignment? That kind of thing was done to evaluate 3dQwarp, as in this poster by recognizable names:
[afni.nimh.nih.gov]
The top part used FMRI data to show the benefits of nonlinear alignment over the older style, linear affine alignment; additionally, the bottom part shows the high quality of 3dQwarp's alignment abilities (separate issue to what you are asking about for EPI motion alignment). But the top part might provide a useful metric.

Looking at motion alignment parameters only can be tough---you want a program to give "the right" motion estimates+alignment; some programs could misbehave by under-aligning, and others by over-aligning, and it would be hard/impossible to distinguish those situations just from motion plots, without seeing the images themselves.

--pt