Hi Paul,
Have a look at
http://www.dxomark.com/ its pretty useful for side-by-side comparisons and even finding lenses you never considered.
On the 400D the Samyang has worse metrics apart from chromatic aberration. BTW I am not sure these are the exact Tokinas you have in mind, as I was unable to compare all three on the Canon 6D (perhaps these Tokinas are only for crop-sensors?).
The Samyang on the 6D looks like it is a pretty sweet combination, perhaps something worth considering if you go to full frame. I have this combination and the distortion is terrible but fixed easily and very well in photoshop. The Samyang is manual focus only.
Let me know if you want to see an example of Samyang during the day before/after distortion/vignetting removal in photoshop.
Rgs Tim.