Beginner learning Cantonese: Start with reading/writing or with speaking?
(As a born Cantonese and not a professional linguist)

Cantonese is mostly spoken while the formal written form of Chinese is always 白話文/語體文 ("Written vernacular Chinese", what I managed to find on Wikipedia).

The written form of Cantonese is informal, and mostly used on Internet forum, text messages, modern subtitle, etc. In Hong Kong, the newspaper "Apple Daily" first used Cantonese in quotes in their articles, they were heavily criticized back then, but Cantonese in formal text is ever so slowly getting to appear in formal occasions.

You might notice there are many 口 radicals in Cantonese, they are mostly invented or borrowed characters as those didn't existed in the past and aren't used in other dialects. https://zh.wikipedia.org/wiki/粵語字. Moreover, many of those aren't standardized because they weren't used in writing that much, like those on the Wikipedia page: 而家, 依家, 宜家, 伊家 means the same in Cantonese.

In Cantonese spoken regions, "Written vernacular Chinese" is always recognized and written Cantonese is not encouraged for formal texts. One can also read Cantonese from Written vernacular Chinese. So unless you know how to speak Cantonese, learning how to read it might not be your first priority. Therefore, for the best effects and usefulness, I suggest first learning how to speak it.



Written vernacular Chinese:

今天晚上我們要去哪兒吃飯呢？


Written Cantonese:

今晚我哋要去邊到食飯？