In the case of en passant, should I move my pawn first or remove my opponent's one? I know that, according to chess rules, every move should be made with a single hand. However, this does not clarify if, when making an en passant capture, I should move my pawn first or remove the opponent's pawn from the board. I understand that the result is pretty identical in both cases, but I wonder what is the preferred consequence if you are playing an international tournament game with strict rules, for example. So, for instance, what do most grandmasters etc. generally do first? (Moving their own pawn vs. removing the enemy pawn.)
Both methods are legal. There is, however, one difference between the two options. If the en passant turns out to be illegal, then you must make a move involving the first piece you touched, if it is legal to so. 

So, if you touch the opponent's pawn first, you must capture the opponent's pawn. If you touch your own pawn first, you must move your own pawn.

I can't say I know which is preferred by grandmasters. But it seems like moving your own pawn first is somewhat less likely to result in disaster if the move turns out to be illegal-imagine having to capture a defended pawn with your queen.