Why is ...e5 dubious here? [fen ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5?!


Why is ...e5 considered dubious for Black here, although it is considered sort of a refutation after the move 5.f3?! e5!

What's the big difference? If it is strong after 5.f3, it shouldn't be so bad after 5.Nc3, the pawn structure is the same.
The problem with e5 is that after 6.Bb5+ white will be able to play the manoeuvre Nf5-Ne3 which will result in an iron control of the d5-square. (Without Bb5 the knight on f5 could be taken by the Bc8). 

The difference to the 5.f3 line is, that f3 doesn't control d5, so when the knight lands on f5 the counterstrike d5 will be possible.

[fen ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5?! 6. Bb5 Bd7 7.Bxd7 Qxd7 8. Nf5