Which is better in the Rossolimo Sicilian, 3... e6 or 3... g6? [FEN ""]
[StartPly "5"]

1.e4 c5 2.Nf3 Nc6 3.Bb5 


In this opening is it better to do e6 to activate the dark square bishop which will protect c5 pawn. If the opponent plays Bxc6, it will have a double pawn in the c file but the dark squared bishop might help the pawn on c5 or g6 because I think the Bishop will be much better on the long diagonal. Also keeping open the option to recapture on c6 with the d-pawn to open the Queen and light squared Bishop. Also not worried about them doubling the pawns because of the Bishop pair to compensate.

I gave my explanation for both moves but which move do you think is Better?
It's really a matter of taste and style. One option may suit more your style than the other. But if you believe in a "moneyball approach", 3...e6 appears to be the "best" move for black based on a search on the 365chess.com website.  
Regarding the number of games, I guess that the difference in black's winning chances (30.2% for 3...e6) versus (26% for 3...g6) should be statistically significant. 

When you control for the level of players by focusing on the Master Database, the statistics also appear to favor 3...e6 over 3...g6.