I watched McCovey play from the time he came up. He wasn't as good overall as Bagwell in my book, but that's per the "eyeball test" a lot of you young guys don't pay much attention to, and that's OK.
Stats are very important, obviously, but they don't tell the whole story. Years ago Casey Stengel was managing a spring training game. A reporter asked him something like, "What do you look for in a ballplayer?" Casey directed his attention to the pregame warm-up and said, "You see those two shortstops out there? They're both 20 years old. In 20 years the one on the left has a chance to be in the hall-of-fame. The other one has a chance to be 40." Casey didn't know how quantify greatness, but he knew it when he saw it.
I'm all in favor of advanced metrics or any other tool that can help evaluate players but I contend there still remain elements to a player that can't be quantified by numbers. McCovey was good but as an all-around player Bagwell was better.
Greenberg was before my time, so I can't say.