Different forms of the sneezer praising Allaah and responding to the one who says “May Allaah have mercy on you” have been narrated. 

Al-Bukhaari (6224) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “When one of you sneezes, let him say, ‘Al-hamdu-Lillaah (Praise be to Allaah),’ and let his brother or companion say to him. ‘Yarhamuk Allaah (May Allaah have mercy on you).’ If he says, ‘Yarhamuk-Allaah,’ then let (the sneezer) say, ‘Yahdeekum Allaah wa yuslihu baalakum (May Allaah guide you and rectify your condition).’” 

Imam al-Bukhaari narrated in al-Adab al-Mufrad (5033) from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “When one of you sneezes, let him say, ‘Al-hamdu-Lillaah ‘ala kulli haal (Praise be to Allaah in all situations),’ and let his brother or companion say to him, ‘Yarhamuk Allaah (May Allaah have mercy on you).’ If he says, ‘Yarhamuk-Allaah,’ then let (the sneezer say, ‘Yahdeekum Allaah wa yuslihu baalakum (May Allaah guide you and rectify your condition).’”

Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

Abu Dawood (5031) and al-Tirmidhi (2740) narrated that Saalim ibn ‘Ubayd (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) said: “When one of you sneezes, let him say, ‘Al-hamdu-Lillaah Rabb il-‘Aalameen (Praise be to Allaah the Lord of the Worlds),’ and let the one who responds to him say, ‘Yarhamuk Allaah (May Allaah have mercy on you),’ and let him say, ‘Yaghfir Allaahu lana wa lakum (May Allaah have mercy on us and you.”

Classed as da’eef by al-Albaani in Da’eef Abi Dawood. But he classed it as saheeh in Saheeh al-Adab al-Mufrad (715) with a mawqoof isnaad that stopped at ‘Abd-Allaah ibn Mas’ood (may Allaah be pleased with him). 

It was narrated that Abu Hamzah said: I heard Ibn ‘Abbaas saying when someone said ‘Yarhamuk Allaah (May Allaah have mercy on you)’ to him: “ ‘Aafaana Allaah wa iyyaakum min al-naar, yarhamukum Allaah (May Allaah save us and you from the Fire; may Allaah have mercy on you).”

Classed as saheeh by al-Albaani in Saheeh al-Adab al-Mufrad, 955. 

Maalik narrated in al-Muwatta’ (1800) from Naafi’ that if ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) sneezed and it was said to him, “Yarhamuk Allaah (may Allaah have mercy on you),’ he would say, “Yarhamuna Allaahu wa iyyaakum wa yaghfir lana wa lakum (may Allaah have mercy on us and you, may He forgive us and you).” 

Al-Nawawi said in Sharh Muslim: 

Al-Qaadi said: The scholars differed as to how one should praise Allaah or respond, and there are varying reports concerning that. It was said that he should say, “Al-hamdu Lillaah (praise be to Allaah)”, or “Al-hamdu-Lillaah Rabb il-‘Aalameen (Praise be to Allaah the Lord of the Worlds)”, or “Al-hamdu-Lillaah ‘ala kulli haal (Praise be to Allaah in all situations)”. Ibn Jareer said: He has the choice between all of these. This is the correct view, but they are agreed that he is enjoined to praise Allaah. 

He said: And they differed as to how the sneezer is to respond to the one who says “Yarhamuk Allaah.” It was said that he should say, “Yahdeekum Allaah wa yuslihu baalakum (May Allaah guide you and rectify your condition),” or that he should say, “Yaghfir Allaahu lana wa lakum (May Allaah forgive us and you).” Maalik and al-Shaafa’i said: He has the choice between these two options. This is the correct view, and the ahaadeeth which mention them are saheeh. End quote 

Conclusion: The phrases used to praise Allaah have been narrated in various forms: 

Al-hamdu Lillaah (praise be to Allaah)

Al-hamdu-Lillaah ‘ala kulli haal (Praise be to Allaah in all situations)

Al-hamdu-Lillaah Rabb il-‘Aalameen (Praise be to Allaah the Lord of the Worlds) 

The phrases used to pray for mercy for the sneezer have also been narrated in various forms:

Yahdeekum Allaah wa yuslihu baalakum (May Allaah guide you and rectify your condition)

Yaghfir Allaahu lana wa lakum (May Allaah forgive us and you)

‘Aafaana Allaah wa iyyaakum min al-naar, yarhamukum Allaah (May Allaah save us and you from the Fire; may Allaah have mercy on you)

Yarhamuna Allaahu wa iyyaakum wa yaghfir lana wa lakum (may Allaah have mercy on us and you, may He forgive us and you) 

All of these are saheeh and the Muslim may choose whichever of them he wants. 

And Allaah knows best.