OUATITW is the better movie in terms of scene composition, cinematography, score. TGTBATU is a great pulp western, but they're just completely different types of movies. One is very much a comic booky-type pure Spaghetti movie with a lot of great action setpieces, the other is sort of an ode to the end of the mythical West, filled with intertextual references to classic Westerns.
It's kind of weird to compare them since they're really different kinds of movies tho they're both Westerns.