Which script should I use to learn Hindustani- Devanagari or Nastaliq?
From what I've heard, both Hindi and Urdu seem to have numerous similarities on a conversational level but both scripts and some more academically inclined vocabulary are super different. I'm also into poetry and I was hoping to write some in the language but was wondering which script to use in the first place...
Sorry if this was confusing but here's the deal: should I lean towards learning Hindi (Devanagari) or Urdu (Nastaliq)?
The differences are bigger than that. There's an entire constellation of mundane vocabulary that shifts as well. For example, greetings are largely religious in India, so you say a different greeting depending on the religion of the person you are talking to. Namaste is a Hindu greeting, while Sikhs are greeted with Sat-sri-akal. Muslims are greeted with Asalam walekum, and say goodbye with Khuda Hafiz, and since Urdu is generally used among Muslims, that is the vocabulary you would naturally learn.
Especially in the 20th Century, there has been a broad push to restore the use of sanskritik terms to Hindi, replacing the Arabic or Persian terms in Urdu, so there will be major vocabulary differences, and while most people would probably understand you either way, they would also be able to tell.
So, the difference between Hindi and Urdu is much more than just Devanagari versus Nastaliq. If you are primarily interested in the Hindu parts of India, then stick with Hindi. If you are interested in Pakistan and the Islamic parts of north India, like UP and J&K, then Urdu will come in handy.
At the same time, if your interest is in historical texts, then Nastaliq is the way to go. Due to the long standing Hindu prejudice against writing, most historic texts in Hindi/Urdu are written with Nastaliq. Prior to the 20th Century, Devanagari was mostly used in writing the Pakrits, and other older vernacular languages. Many of those languages have evolved into their own languages today, such as Nepali, Panjabi, Odia, etc.
Going on the fact that you are poetically inclined, I recommend Urdu.
It's a linguistic thing that nomadic languages are much more poetic, expressive and deeper than normal languages. Arabic - the most popular nomadic language - and Urdu, share a similar script and both are poetic (although they do have differences). Urdu has more classifications of literature and poems each (more than just the sonnets and ballets) indicating its richness in literature. It also merges with languages like Persian and Farsi - known for their calligraphy and literary sense.
But the best part? Learning Urdu gives you a taste (if not the best) of both worlds and cultures that are so different - Hindi and Arabic. Why is that good for you? You want to learn the Hindustani vocabulary (Hindi) and you are poetically inclined (Arabic) - both qualities found in Urdu.
And once you have learnt Urdu, learning true Hindi or Arabic will be so so much easier, and just like that, you can know 3 languages! (You don't get such advantages if you chose to learn Hindi alone.)
I say this only because I know all 3 languages.
PS - If you had like, could you tell us what you have decided?
Honestly, you should learn both. A lot of poetry, until the middle of the 20th century, was written in Nastaliq whether by Hindus or Muslims, but Devanagari predominates in India by far today, is used in vernacular newspapers and prose, as well as Sanskrit, Nepali, and Marathi works. Nastaliq is the dominant script in Pakistan. It would be reasonable to learn them together, and match up the Devanagari and Nastaliq symbols for the same sounds when possible.