Aaron was more consistent, but Mays' highs were greater.
Mays was faster, a superior defender, and played his home games in two very hitter unfriendly ballparks. If Mays had played in County Stadium and Fulton County Stadium, instead of the Polo Grounds and Candlestick Park, he probably would've been the one to break Ruth's record. I once saw a metric called Venue-independent hitting. I have no idea how it's calculated, but Willie Mays was number one on the list for overall hitting.
As it was, Mays had a higher slugging pct, a higher On base pct, and thus a higher OPS. Those are the batting standards that are used to judge an offensive player. His WAR is higher despite playing one less season, and playing a couple seasons when he was clearly washed up. On top of that, he lost two seasons during his prime, serving in the Army.
Both were all time greats. I was a fan of both when I was growing up. But, looking past the raw numbers, and using the advanced metrics used to compare players, Willie Mays could be the greatest all around ballplayer.
These type of comparisons are always fun. Good question Hank.