Yesterday, I have come to the conclusion, that if you want a full circular fish-eye image, you should use a full circular fish-eye.
On might claim that the 10.5 is sharper than the 8mm Sigma but I believe you lose a bit of image quality when merging the two images made by the 10.5. Also, the disadvantage is that you concentrate so hard on making the two without changing the perspective that you don't concentrate on the composition itself.
So no, I will not replace the 8mm Sigma with the 10.5DX.
You can make a circular image with the 10.5DX but with some drawbacks, including moving subjects in the frame when exposing twice, and last but not least, you need additional time in post processing to achieve something you could have with just one click of the shutter release