We don't know that. For better or worse---and I personally would rate CMU's CS undergraute education as superior to MIT's now, but that's because EE and MIT's vision for EECS graduates does little for me---MIT can attract a significantly better pool. E.g. before applications got really crazy in the Great Recession, when 13,000 were applying, it thought 3,000 "could do the work" and chose from that subset.
And MIT doesn't do legacy admits; I get the impression CMU may.
And MIT doesn't do legacy admits; I get the impression CMU may.