Lauri Torni wrote on 08/15/17 at 09:08:06:
3.-Nf6 is positionally rather fragile. I think this is the reason for the popularity of 3.-c5 at high level (- c5 is also my choice).
Yes, the variations lead to quite different types of positions. Let's look at two tabyias which represent a main line of each variation:
In the 3...c5 line, White is ahead in development, his king is safer and he can quickly develop some threats against Black's uncastled king. Although Black lags in development, one pair of knights has been exchanged, his pawn structure is sound and he has an extra central pawn so his long term prospects are good if he can mobilize while neutralizing White's attacking chances.
In contrast, in the 3...Nf6 line, both players are similarly mobilized although White has succeeded in castling and his king is safer. Obviously Black's pawn structure is weaker with 3 pawn islands to 2 for White and his backward e-pawn (although he does possess 2 center pawns to 1 for White.) . Generally speaking, Black must play more dynamically in order to take advantage of his attacking chances in the middlegame since passive play would ultimately favor White's better pawn structure. Indeed, the thematic exchange sacrifice ...Rxf3 is often an important resource for Black utilizing the half-open f-file.
Both positions have their merits and disadvantages for Black. At the risk of over generalizing, the 3...c5 tabiya should appeal to solid, positional players who don't mind defending and are looking to outplay their opponent over the long term. I would also assume it appeals at the highest levels where solid technique is the norm and a draw is an acceptable outcome as Black. On the flip side, the 3...Nf6 line requires more dynamic, fighting play by Black (it sort of reminds me of playing the Black side of the King's Indian Defense where I feel the need to generate pressure against White without allowing him to drift into an advantageous endgame.)
Just a few comments to stimulate discussion!
By the way, in my sample of 2400+ players, Black scored the following in each tabiya:
3...c5 tabiya Black scored 5W - 17 D - 10L
3...Nf6 tabiya Black scored 2W - 3D - 3L