I fully understand that runs were more scarce in Mantle's era, but I think the difference is TOO MUCH to excuse just by saying Gehrig had better conditions to hit...especially in average, plate discipline(putting contact on the ball) , and RBIs. Both played on the best offenses of their eras. Also their QUALITY may have been similar, when they did play, but Gehrig was putting this quality into FULL SEASONS, he wasnt missing a lot of time each year, like Mantle. You must give him credit for being in there every day.
Lets put it this way, IF Mantle had stayed healthy and never gotten hurt, his career would be much like Gehrig's. Say if he had 8 or 9 seasons like 1956. Mantle is more a 'what if?' kinda guy, and Gehrig is someone who actually fully realized the full potential of just how good a hitter truly can be(until he got the disease). Give Mantle an edge in fielding and running, but if his running didnt actually do anything to help him get more stolen bases and extra base hits, then i dont see how it can count for that much. Maybe he could have gotten a lot more infield singles and hit .350, but if Gehrig could hit .350 without the infield singles then obviously that negates any speed advantage.
I think a much beter comparison is Mantle and Frank Thomas..their rate stats are nearly identical, mantle a bit more power, Thomas more of a run producer. They both had awesome peaks, but maybe Mantle a bit better. But Thomas had more great seaons where he stayed healthy and put it all together. Obviously Mantle gets the edge due to all around play, but just for pure hitting i see them as about equal.