According to Diamond Dope, the Babe's cards correlate closely in offensive value with their cost. So the most expensive card, 1923, is going to give you the most offense.
That said, it seems to me that the best value comes with the 1920 card. I'd use that one up to and probably including 140M leagues--though I might go up to the 1921 card in a 140M if I could spare the cash.
I'd save Ruth's most expensive 1923 card for 200M leagues. At that price level, sheer performance is more important than value.
BTW, I did nominate the 1923 for inclusion some years ago, and it was voted in. So I do have a warm feeling for that card, and like using it when I can draft Ruth (not easy) and when I can afford it. I believe it's the best card in the set.