I’d rather watch the movie first because you get familiar with the characters and can then learn more with the book. I have aphantasia, which is when I read detail, it is hard for me to imagine what characters and places look like. So, when I watch the film first, I get an actual face to visualise rather than having to build the character from the facts in the book. The film therefore helps with visualising the characters and scenes in the books when you read it later. Mostly the films only offer a surface level of the character and storyline, so then when you read the book later, you get a much deeper understanding of the character as a person and plot.
Films are also only the director’s interpretation of the texts so often things are missed out or removed completely. For example, in the book ‘Little Women’ the women wear bonnets but, in the 2019 version of the film, they don’t. Not only is this historically inaccurate, but it also misses out crucial parts of understanding the story. In one part of the story in particular, Beth gets sick, and no one moves her bonnet until she gets better. The bonnet is an important symbol as removing this was the last thing Beth did before she got sick. The film completely missed this part out, which is a shame because I think it would have been a good opportunity in the film to show how long Beth was sick and showing time moving on, the weeks and months passing. As this was missed out, you didn’t get the same sense of time, it didn’t feel like Beth was sick for very long and instead it felt rushed.
Also, in film you often don’t get the whole story, especially with endings. Directors seem to prefer to give a happy ending to please the audience rather than giving you the whole story. They will miss out details to keep a certain pace and leave the audience feeling satisfied rather than being true to the story. For example, in Pride and Prejudice, there was an alternative ending for the USA that focussed on Mr Darcy rather than Lydia and her story, they wanted to end on the happy part rather than the more uncomfortable truth.
Overall, I would say it is worth watching the film to help build an image in your mind (especially if you find that difficult like me) and then read the book to gain a deeper understanding about the characters and story lines.