Differences Tamron f/2.8 28-75 / 24-70
Yes, there is a lot you are missing.  

The 24-70 has substantially better transmittance (more light makes it through the lens), slightly better sharpness, significantly less chromatic aberration, 2 more diaphragm blades (better, more round bokeh), full time manual focus (can auto-focus at the start of a shot but still adjust after starting shooting).  

Also, image stabilization can be very helpful when shooting video.  If you are on a tripod you shouldn't, but if you are shooting free-hand, it is a good thing unless you need quick, lag free panning (which would be a problem due to your rolling shutter on a 600d anyway).

While I don't recommend putting too much stock in the overall score that DXoMark produces, there individual discreet stat comparisons are generally reasonably good.  You can find the comparison for these two lenses on the 700D listed here.  (Sadly, they did not have data for the 600D specifically.)