I have both MB4-T and MC-11 adapters. Bought MC-11 about one year ago on special $150 price. I have not owned any Sigma EF-mount Art or Sport lenses yet that could work slightly better with Sigma own MC-11.
With all latest FW updates, they perform similarly on my Canon lenses in most scenarios. However I feel eye-AF on A9 with MB4-T under green mode seems perform slightly better.
My legacy Canon 500L/4.0 IS super-tele lens only works with MB4 under the advanced mode on A9 that CDAF is fast and still can AF under very dim light, and can use all AF points, while MB4 green mode and MC-11 only can use AF points in 1/3 center area under PDAF and unable to lock under very low light.
MB has tripod mount while MC-11 doesn't. I mount this
RRS L-bracket on Metabones that works very well (a bit bulky) that 17L TS-E can fully T/S out without bumping with a regular L-bracket mounted on camera body. EF 24-70L/2.8 II is another relative heavy lens without own tripod collar.