Once Upon a Time in the West is far superior.
I saw both movies back in the 60's. I was not impressed with GB&U back then. I was mesmerized by OUATITW. I watched both movies recently and I felt GB&U was so contrived and ridiculous, where as OUATITW has held up quite well and stands the test of time. Even Clint Eastwood said GB&U was "bloated".
The scenery in GB&U and the sets however were fantastic.