Hungarian Defence
Both Nf6 and d6 are good moves in this position, and both have been played in top level games
I feel like if you had to pick one against Karpov, either would do. But against fellow amateurs, Nf6 is probably more versatile. Against anyone who knows Hungarian theory, your next three moves are probably Nf6, d6, and 0-0, in some order. But against schmucks every bit as likely to play a3 or something on the next move, it's nice to have the option of a quick d5 held back a move or two.
e.g. - ...Nf6 5.a3 Nxe4 6.Nxe4 d5, with the fork trick.
I feel like if you had to pick one against Karpov, either would do. But against fellow amateurs, Nf6 is probably more versatile. Against anyone who knows Hungarian theory, your next three moves are probably Nf6, d6, and 0-0, in some order. But against schmucks every bit as likely to play a3 or something on the next move, it's nice to have the option of a quick d5 held back a move or two.
e.g. - ...Nf6 5.a3 Nxe4 6.Nxe4 d5, with the fork trick.
Who would play 5.a3??