The differences in obstruction would only be significant if the optics were equally good. Thermal management of such a long tube ( which should be made oversize as it is open ) could be significant as is the 2" thick mirror . On top of that the quality of the optics is a bit of an unknown quantity.
You would need tracking too for the long dob - always fairly expensive ( allow $2500 or so for a servo-cat and argo.)
Given all those problems my money would be on the C11- you sit down and observe and the tube should be much more stable in a breeze. Also when you inevitably get into planetary imaging with a webcam
the Sphinx mount will be very precise for that .