Which is better: standing for a long time or doing more rak‘ahs?

The scholars differed concerning this issue, and there are three views.
The majority of Hanafis, the Maalikis – according to one view – and the
Shaafa‘is, and it is also a view held by some of the Hanbalis, say that
standing for a long time is better than increasing the number of rak‘ahs. 

The Maalikis – according to the view they regard as more correct – and some
of the Hanbalis say that it is better to do a lot of bowing and prostrating
(rukoo‘ and sujood). 

The Hanbalis have a third view, which is that the two matters are equal,
because of the conflicting reports concerning that. 

Among the things quoted as evidence by those who say that standing for a
long time is better is the hadith of al-Mugheerah ibn Shu‘bah (may Allah be
pleased with him), who said: The Prophet (blessings and peace of Allah be
upon him) used to stand to pray for long that his feet – or his legs – would
swell. Something would be said to him, and he would say: “Should I not be a
thankful slave?” 

Narrated by al-Bukhaari (1078) and Muslim (2819). 

One of the things quoted as evidence by those who say that doing more
rak‘ahs is better is the hadith of Abu Hurayrah, according to which the
Messenger of Allah (blessings and peace of Allah be upon him said: “The
closest that a person is to his Lord is when he is prostrating, so say a
great deal of du‘aa’.”

Narrated by Muslim (482). 

One of the things quoted as evidence by those who held the third view, which
is that they are both equal, is the report narrated from Hudhayfah, who
said: I prayed with the Prophet (blessings and peace of Allah be upon him)
one night, and he started to recite al-Baqarah and I thought: he will bow
when he reaches one hundred, but he carried on. Then I thought that he would
finish it in the two rak’ahs, but he carried on. Then I thought he would bow
after finishing it, but he started to recite al-Nisa’ and recited it all,
then he started to recite Aal ‘Imraan and recited it all, reciting with a
slow and measured pace. When he reached a verse that spoke of glorifying
Allah, he glorified Allah; when he reached a verse that spoke of asking of
Him, he asked of Him; when he reached a verse that spoke of seeking refuge
with Him, he sought refuge with Him. Then he bowed and started saying, ‘Subhaana
Rabbiy al-‘Azeem (Glory be to my Lord the Almighty).” And his bowing was
almost as long as his standing. Then he said: “Sam’a Allahu liman hamidah
(Allah hears those who praise Him).” Then he stood for a long time, almost
as long as he had bowed. Then he prostrated and said, “Subhaan Rabbiy al-A‘la
(Glory be to my Lord Most High),” and his prostration was almost as long as
his standing.

Narrated by Muslim, 772 

Shaykh al-Islam Ibn Taymiyah regarded the third view as most likely to be
correct. He (may Allah have mercy on him) said: 

The people disputed as to whether it is better to stand for a long time or
to do a lot of bowing and prostrating, or are they both equal? There are
three views, the soundest of which is that they are both equal. When on
stands (during the prayer), one recites Qur’an, which is better than dhikr
and du‘aa, but prostration in and of itself is better than standing. So if a
person stands for a long time, he should also make his bowing and
prostration lengthy. This is the lengthy qunoot which the Prophet (blessings
and peace of Allah be upon him) mentioned in his answer when he was asked:
Which prayer is best? He said: “Lengthy qunoot.” Narrated by Muslim (756).
Qunoot means spending a long time in worship, whether that is when one is
standing or bowing or prostrating, as Allah, may He be exalted, says
(interpretation of the meaning): “Is one who is obedient (qaanit) to
Allah, prostrating himself or standing (in prayer) during the hours of the
night…” [az-Zumar 39:9]. Allah describes him as qaanit (translated here
as obedient) when he is prostrating, just as He described him as qaanit when
he is standing.

Al-Fataawa al-Kubra
(2/121-122) 

And he said: 

The scholars disputed as to which is better: standing for a long time or
doing a lot of bowing and prostrating? Or are they both equal? There are
three views, all three of which were narrated from Ahmad. It is proven from
him (the Prophet (blessings and peace of Allah be upon him)) in as-Saheeh
that he was asked: Which prayer is best? He said: “Lengthy qunoot”; and it
was proven that he said: “you will not perform one prostration to Allah but
Allah will raise you one degree in status thereby and erase one sin for you”
Narrated by Muslim (488). And he said to Rabee‘ah ibn Ka‘b (who wanted to be
with him in Paradise): “Help me to do that for you by prostrating a great
deal.”. Narrated by Muslim (489). 

It
is well-known that prostration in and of itself is better than standing, but
the dhikr of standing is better, because it is recitation of Qur’an. Thus we
may conclude that what is best with regard to prayer is to be consistent. So
if you make the standing lengthy, you should also make the bowing and
prostration lengthy, as the Prophet (blessings and peace of Allah be upon
him) used to do when he prayed at night, as was narrated by Hudhayfah and
others. This is how his obligatory prayers were, as well as the eclipse
prayer, and others. His prayer was consistent, so if someone prefers to make
his standing, bowing and prostration lengthy, with fewer rak‘ahs, or to keep
the standing, bowing and prostration shorter whilst doing a lot of rak‘ahs,
these two matters are close in virtue, and the latter may be preferable in
some situations. When the Prophet (blessings and peace of Allah be upon him)
prayed Duha on the day of the conquest of Makkah, he prayed eight brief
rak‘ahs, and he did not limit it to two lengthy rak‘ahs. The Sahaabah also
did that when praying qiyaam in Ramadaan, so as not to cause hardships to
the people praying behind them by making the standing lengthy. 

Al-Fataawa al-Kubra
(2/252). For some important details, please see Majmoo‘ al-Fataawa
(23/69-83). 

And Allah knows best.