630 homers and ten gold gloves put him at a place with Willie Mays alone. A guy that missed tons of games due to injury and a strike still ended up putting up legendary numbers. Throw in no steroid controversy in the dirtiest era of baseball, and you got a case for Ken as the best.
When Griffey first came into the big leagues no one had seen anything like him. Bonds was more consistent and had more awards, but also probably did more steroids. There was nothing artificial about Griffey's talent. His swing was and still is legendary, he could have stolen more bases than he did, and he is arguably the best center fielder ever.
Yes, Ken Griffey was the best position player in baseball during the 90's. You could also make a very strong argument that he was the best player overall during that decade. He was great in the outfield and a feared hitter. Ken Griffey will make it to the Hall of Fame off of the strength of his 90's play.
Yes, Griffey played center and Bonds left, but Griffey never won a GG after moving to the NL. Bonds was a 300-300 (HR-SB) player for the decade and won 3 MVPs. Whatever edge you want to give Griffey defensively, Bonds made up for with offense. Also, Bonds didn't get the luxury of hitting in the Kingdome or DHing a few times a season.
In order to say someone was the "best" we need to figure out what that means. What exactly does it mean to be the best position player? How can we quantify that? Once we can figure out exactly what stats to use to quantify who is the best player at any given moment, then we can answer this question without it being an opinion.
Barry Bonds was the best position player of the 90's. He won three MVP awards to Griffey's one, and it's likely he didn't start doing steroids until after the year 2000. During the 90's, Bonds posted higher numbers in OPS and WAR. Both players played equally well in the field.