The c3 sicilian

Why is the c3 sicilian considered to be a semi-respectable anti-sicilian, played by Sveshkinov and Hess, among others, while the Ponziani is considered to be a terrible opening? 

the ponziani isnt terrible

c3 sicilian is semi-respectable only at club, or sub master level.

The 2. c3 or Alapin Sicilian is used by GMs, IMs and.NMs to draw against higher rated players. And crush equal and lower rated players. A must learn variation by Sicilian players.

Did GM Hess not get the memo when he beat GM Negi with it?

http://www.chessgames.com/perl/chessgame?gid=1628297

Just because a GM plays a line, that doesn't mean it's a good move. 

GM Miles played 1. ... a6 against Karpov, a former world champion. Is 1. ... a6 a good move?

Having said that... most lines are at least marginally playable. Even at the GM level, it usually takes more than one error to lose the game.

Miles played 1... a6 one time. Sveshnikov has played the c3 sicilian 111 times in the database.

But the question isn't really whether the c3 sicilian is a great chance at an advantage. The question is, why is the c3 sicilian semi-respectable as an anti-sicilian, while the Ponziani is practically never played and almost universally regarded as garbage?

Потому, что в Варианте Алапина мы подрываем пешку с 5 а в Варианте Понциани мы тратим темп.

Sorry, I only understand the english language

It's kinda interesting that Fabiano Caruana himself called the c3 sicilian a good opening, while he was commentating on his game from round 3 of the ongoing US chess championship. He said: " today I was struggling from the opening. He had this clever opening choice playing c3 sicilian." End of quote. He was playing against Awonder Liang. The interview is on Youtube.

The Ponziani and c3 Sicilian don't have much in common so it's not fair to compare them. There is a lot of theory in the Ponziani, and the opening was for some time considered inferior, although I think that that assessment has changed now that computers have shown more openings are playable than previously known.

What makes the Alapin semi-respected? 2. ...d5 ?

If by "sub-master" you mean Kasparov, Short, Polgar, etc. They've all played it with white from time to time

What I actualy meant is that it is mostly amateur players who consider it an inferior and/or dubious opening, not professional players.

The Ponziani allows black to seize the initiative by means of d5! or Nf6!?. This opening tries to salvage a wrong strategy via the use of tactics. I will not give a direct line as refutation but after Qa4, f6 avoids all tactical nuances.

The Alapin on the other hand attempts to achieve the strategic plus forming a central pawn phalanx which would neutralize the positional edge that the better pawn structure gives black in the open sicilian. Black's best try is Nf6 which allows for quick piece development for white and a rich equal middlegame.

The ponziani is not a mistake but it is clearly a sub-par choice.

The Ponziani gives black the easiest game out of all aceptable moves that follow: e4 e5, Nf3 Nc6. That is why I call it sub-par it gives black the fewest problems out of the 5 big (Ruy Lopez, Italian, Scotch, 4 knights, Ponziani).

At the end of the day all mistakeless paths lead to equality but there are clearly more challening paths to lead black into.

Which would you suggest gives black the easiest game after: e4 e5, Nf3 Nc6 if not the ponziani? (talking about respectable openings which the ponziani clearly is)

Carlsen won a fantastic game vs Harikrishna using the Ponziani. 3...Nf6 gives white good chances even when is equal.The problem is that if blacks decides to play 3...d5 there is an immediate equal endgame. I don't know if in the Alapin black can get an instant endgame as well.

I agree with you on it being the sharpest option (maybe the evans is sharper but it can be avoided) but it still believe that f6 neutralizes the attack

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