I'd strongly advise
not associating Baha'u'llah with The Bab. Baha'u'llah screwed over his own brother (Subh-i-Azal - the appointed successor) as the opportunist he was, when he was not the appointed successor that The Bab himself directly appointed in person and in writing, and was not "he whom God shall make manifest" either, who is still over approx 1000 years away as of this year according to the writings of The Bab.
Baha'u'llah just decided that 25 years later "he whom God shall make manifest" would come not even 1% of the prophesied time of the Babi/Bayani dispensation. It's hilarious though.
The Bab though, as far as who he was and what he taught. He was an occultist, and a marvelous one at that.
If one accepts the Shi'i view, then The Bab is clearly an authentic form of restorationist (at the very least) who through his teachings united the Twelver stream back with the Ismaili one (Twelve Imams + the hyper-eSoteric occult philosophy innate in the Shi'i worldview) - so circa Imam Jafar al-Sadiq era restoration.
From the point of view of someone of that disposition with Shi'i leanings, I can strongly respect and love the Bab on those terms.
On revelatory terms, it's still very contentious. Muhammad is the last of the Prophets (Surah 33:40, plus endless Hadith), it doesn't get much clearer than that. Yet such an idea does have it's own internal contradiction which could be logically exploited for such defense.
Either way, God, according to the Qur'an, does not leave humanity at any time without an Imam, or guide, or Khalif (various Quranic passages and Hadith mention it), so there is always the question mark there. In the Shi'i view, obviously it either means the Mahdi in Ghayba or the Aga Khan for Nizaris.
It's worth noting though, that since Baha'i's falsify the belief in the Twelve Imams, the Mahdi in particular, and take the Sunni view instead, they deny the epistemology of their own truth claims which comes out of the Shi'i camp not the Sunni one.
Anyway, the Bab's teachings are very profound, unfortunately not enough has been translated yet and Baha'i's don't want to risk translating any of his writings to english (it's the last thing on their list).
This site is fantastic though, you may enjoy it:
Haykal - Writings of the Bāb | Hurqalya Publications: Center for Shaykhī and Bābī-Bahā’ī Studies
The book "The Messiah of Shiraz" by Dennis MacEoin has a lot of fantastic insights as well, although not detailed enough in parts.
Something that makes no sense, in the Baha'i view. The Babi/Bayani view differs though, particularly in the realm of esotericism.
What remains though is that in terms of scripture itself, The Qur'an is still as it was in Prophet Muhammad's time, there is no comparison to the situation of the Bible predating Islam. Same thing goes for Islam's universally normative practices, still done as it was back in the Prophet's time (5 pillars et al).
Also one thing I'll call to mind is the nature of scripture as well. The Qur'an, regardless of one's opinions about it, sees itself as and speaks as God's direct word the whole way though (sparing the metaphysical semantics alluded to with the preserved tablet, Umm al-Kitab etc). The Qur'an is the only scripture like this, and has also been consistently memorized since the Prophet himself received the revelation.
In some of the Bab's writings it goes in and out of perspectives, sometimes it is the Bab himself speaking, other times the Imam Mahdi, other times it is God. Depends on the sacred text of his and the nature of that text. Whatever work of the Bab you read though, it's always very symbolically dense and takes lots of rereading to grasp all of what it's saying, because it's again: esoteric. Everything he does with symbol, Tawil, the arabic letters, etc, is all incredibly inventive and very true to the science of the letters ('ilm al-Huruf) taught by the Ahl al-Bayt.
But as for Baha'u'llah, aside from Quranic allusions, it is rather usually him alone being the speaker (exception of certain poems). Baha'u'llah's writing style and his basic nature as a "holy figure" (which I contest of course) reminds me heavily of the "apostle" Paul of the New Testament. He is a very Paulian figure, and curiously a usurper like Paul was (Paul usurped James the Just - aka Jesus's brother).
He writes epistle-style works, even for his non-letters.
There aren't many of his works that aren't this.
His writings aren't very symbolically deep, most of it superficially borrows aspects from the Babi/Bayani oeuvre, with bits of basic Islamic and Christian symbol, in other areas he borrows Sufi flourishes (while also denying mysticism).
Later Baha'i authorities, namely Abdul'baha and Shoghi Effendi, are the ones who really create "the Baha'i faith" our of the blueprints of his writings and try to de-Islamicize his work, and that of The Bab (who is of course treated as only a footnote in Bahaism).
From a legalistic standpoint, there isn't much to say there. The Bab is a more complex figure regarding his idea of abrogation. One thing is certain though, Baha'u'llah returns to medieval punishment, whereas The Bab doesn't. It's quite a strange thing. Baha'u'llah's laws, except for doctrinal stuff, are not implemented yet by Baha'i and no wonder why.