Although the fourteen infallibles are all at the pinnacle of perfection and spiritual completion, yet the holy prophet (pbuh) of Islam is the highest of them all, possessing advantages and characteristics that no other infallible shares with him, as Imam Sadiq (as) says in the same hadith you are asking of: “The imams are all like the messenger of Allah (swt), the only difference being that they aren’t prophets, thus what is halal for the prophet isn't halal for them (they can't have more than four wives), but they are like him in everything else.
The fourteen infallibles are fourteen shining stars at the highest point of spiritual ranks and human perfection. All of them possess divine knowledge and perfection, while deficiency and misguidance have no way in them: “Verily, Allah's will is to remove away impurity from you, the people of the messenger's household and to purify you a thorough purification”. So all of these glorious individuals are at the climax of human perfection and salvation.
Nevertheless, the messenger of Allah (swt) is the highest and most complete of them all; possessing characteristics and advantages that no other infallible shares with him, as Imam Sadiq (as) has stated in the same hadith you have questioned about: “The imams are all like the messenger of Allah (swt), the only difference being that they aren’t prophets, thus what is halal for the prophet isn't halal to them (they can't have more than four wives), but they are like him in everything else.”
Imam Sadiq (as) has been narrated saying that the prophet of Islam was the first to answer Allah’s (swt) call in the realm of Dhar and was the closest of all to Him and obtained the highest spiritual rank that even none of the special messengers and closest angels to Allah’s (swt) dominion had ever reached. He is so close to Allah (swt) that He [Allah] says in the Quran: “Until he was within two bows’ length or even nearer”. He is the one who ascended the many levels of spirituality, all in one night, during the Mi’raj. Out of all of the infallibles, he is the only one to have received revelation directly from Allah (swt). The Holy Quran was revealed onto him, the most complete religion ever to come from Allah (swt) was given to him and was entrusted with conveying it to the people. He is the one who has reached the highest levels of morality and the only one whom Allah (swt) sends his continuous salawat [blessings and mercy] upon and makes the salawat, the salawat of the angels and all believers solely his.
Each one of these advantages by itself is enough to show how unique and high of a rank the prophet (pbuh) possessed and as Imam Baqir (as) beautifully explains the rank of every infallible: “The “good tree” is the prophet (pbuh)…the branch of the tree is Ali (as), the trunk of the tree being Fatimah (as) and its fruit the children of Ali (as) and Fatimah (as).”
In this valuable narration, the rank of each of the infallibles has been portrayed in a very beautiful manner.
In the end, one thing that needs to be pointed to is that knowing the rank of the infallibles is a very important matter which needs to be thoroughly and correctly fulfilled; both neglecting to do so and going beyond the limits about it are dangerous and are means of misguidance. Imam Ali (as) states in the Nahjul-Balaghah: “Two categories of people will face ruin on account of me; he who loves me with exaggeration and exceeding the bounds [by saying that I am God], and he who hates me intensely.” Therefore, we are obliged to recognize the ranks and importance of the infallibles and follow their footsteps to salvation and felicity along with putting aside negligence and excessiveness regarding them.
Hopefully, we will all by the mercy and grace of Allah (swt) be lead to the straight path and be protected from going astray.
 عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنِ الْحُسَیْنِ بْنِ سَعِیدٍ عَنْ عَبْدِ اللَّهِ بْنِ بَحْرٍ عَنِ ابْنِ مُسْکَانَ عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِی عَبْدِ اللَّهِ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ ع یَقُولُ الْأَئِمَّةُ بِمَنْزِلَةِ رَسُولِ اللَّهِ ص إِلَّا أَنَّهُمْ لَیْسُوا بِأَنْبِیَاءَ وَ لَا یَحِلُّ لَهُمْ مِنَ النِّسَاءِ مَا یَحِلُّ لِلنَّبِیِّ ص فَأَمَّا مَا خَلَا ذَلِکَ فَهُمْ فِیهِ بِمَنْزِلَةِ رَسُولِ اللَّهِ ص Osul Kafi, vol.1, pg.270; Mir’atul-Uqul, vol.3, pg.161.
 انما بعثت لاتمم مکارم الاخلاق Bihar, vol.16, pg.210.
 ان الله و ملائکته یصلون علی النبی یا ایها الذین آمنوا صلوا علیه وسلموا تسلیماً Ahzab:56.
 Referring to the verse: کشجرة طبیه اصلها ثابت و فرعها فی السماء Ibrahim:24.
 Tafsirul-Qummi: أَبِی عَنِ ابْنِ مَحْبُوبٍ عَنْ أَبِی جَعْفَرٍ الْأَحْوَلِ عَنْ سَلَّامِ بْنِ مُسْتَنِیرٍ عَنْ أَبِی جَعْفَرٍ ع قَالَ سَأَلْتُهُ عَنْ قَوْلِ اللَّهِ تَعَالَى مَثَلًا کَلِمَةً طَیِّبَةً الْآیَةَ قَالَالشَّجَرَةُ رَسُولُ اللَّهِ ص وَ نَسَبُهُ ثَابِتٌ فِی بَنِی هَاشِمٍ وَ فَرْعُ الشَّجَرَةِ عَلِیُّ بْنُ أَبِی طَالِبٍ ع وَ غُصْنُ الشَّجَرَةِ فَاطِمَةُ ع وَ ثَمَرَاتُهَا الْأَئِمَّةُ مِنْ وُلْدِ عَلِیٍّ وَ فَاطِمَةَ ع شجرة رسول الله نسبة ثابت فی بنی اثم و فرع الشجرة علی بن ابیطالب و غض الشجرة فاطمه (س) و ثمراتها الائمة من ولد علی و فاطمه (ع) و ... Biharul-Anwar, vol.9, pg. 218.
 “هلک فیّ رجلان محب غال و مبغض قال” Nahjul-Balaghah, Short Sayings, no.113.