I can't say which is better because I haven't used MMO. Some mechanics I know use MMO during the break-in of rebuilt small aircraft engines (Lycoming and Continental).
As for Seafoam, I've been using it since the late 1960's (eeek! I can't be that old!). I started using it to decarbon 2 cycle outboard engines. It worked.
Then when I got into motorcycles, I started using Seafoam to clean up carburetors of bikes that had been sitting unused for months or years. Anything was worth a try to prevent having to disassemble a 4 carb setup. Again, it worked.
There are a multitude of carb cleaning products but I haven't found one that works better. I have also come across some naysayers as to the effectiveness of Seafoam, but not many.
Until I find something better, I'll stick with the Seafoam when needed.