Hi,
I personally prefer to play ...a5 before ...f6. The reason being that in this Tarrasch variation I like to attack on the queenside by means of a5-a6, and the knight on d7 goes to b6. So even though I might not get this exact position I would play ...Qxf6 anyway.
Regards.
After cxd4 the Tarrasch closed main line ends and the move black is eager to play here is f6 to hit that d4 e5 pawn structure. White recaptures with the exf6.
Now I just want to know which do you guys prefer 9.Nxf6 or 9.Qxf6 and why.