I agree with Z00t. Take the extra pawn and follow the line into the endgame. Over the board, there's always a chance you'll make a slip, but in correspondence you have the chance to check databases which have thousands of Marshalls to look at. Of course, if you just bang out moves you'll get in trouble. Take your time. Remember, even Marshall lost his game to Capablanca, the first time it was used in pro chess.