Symphony has a number of additional panels in the color corrector interface that I use constantly. You can make adjustments on Shadows, Midtones, and Highlights independently more easily.
Symphony has powerful secondary correction, where you can have multiple secondaries going at once. They are based on sampled colors though, not user drawn masks or “power windows”.
Symphony can apply color correction based on source tape automatically, and can also apply an adjustment or even a completely new grade on top of an existing color correction.
This is just a really quick, off the top of my head list. Yes, Symphony has a better and more powerful color corrector. Whether its worth several thousand dollars more than MC, when you can get DaVinci Resolve essentially for free, provided you have the necessary hardware, is your call.