From ePhotoZine's test:
From looking at their graphs, it would appear that resolution is higher wide open in the Tamron at the 70 and 135mm range, but the Canon is higher at 200mm.
The chromatic aberration is slightly lower in the Tamron, although both are negligible in field conditions.
Fall-off illumination numbers are lower on the Tamron than the Canon.
1.85 stops Tamron @70mm 2.09 stops @200mm
1.92 stops Canon @70mm 2.6 stops @200mm
Lower distortion on the Tamron too:
From the Canon review: "Distortion is minimal at 70mm with Imatast detecting 1.96% barrelling. At 200mm 1.12% pincushion distortion is present, which is a moderate level, but may be noticeable under certain circumstances. At both ends of the zoom the distortion pattern is uniform across the image area, which should make it simple to correct in image editing software afterwards. "
From the Tamron review: "Distortion is very well controlled throughout the zoom range. At 70mm only 0.645% barrel distortion is present, which is replaced with 0.42% pincushion distortion at 200mm. If straight lines are paramount, then you'll be pleased to learn that the distortion pattern is uniform across the frame, making it relatively easy to correct in image editing software afterwards, although this distortion is so mild, very few people will actually need to apply any corrections."
This looks like it is shaping up to be a pretty impressive lens.