East and West are a little different. But North and South are VERY different. There are southern accents that are almost impossible for people from the north to understand.

Hi, Salvatore! Overall, there are some differences between the American East Coast and West Coast Accents. However, it gets more difficult than that. Let's look at your example of New York State. There are numerous accents within that one state. Same with California. There are several different accents, each according to education, social class, region of the state, etc. Not to mention numerous immigrant groups who have all contributed their own accents and vocabulary to these two regions. So, your question of which is the most difficult to learn is impossible to answer. It depends on the learners native tongue, for one thing, and how they produce sounds in their own language. This is why Accent Coaches like myself tend to teach a non-regional neutral accent known as the Standard American Accent. It's the accent you usually hear on American TV shows and movies. It is neutral because it does not make the speaker sound like they are from a particular region, and it sounds educated and clear. It's a great focus for English learners! I had to learn it for professional advancement! If you'd like to talk with me, set up a Trial Lesson and I can give you some insights on your English Accent and a few things you should focus on for quick improvement. I do something called "Quick Fix" Training that my private students have loved, and I'm happy to offer it here on italki!

Hi, I’m a native English speaker and yes the accents are completely different. But the east coast also had a lot of variety. Tv mostly shows the generic ny/nj/Boston accents but there’s way more. English spoken in California vs New York is the same but they do have different slang.

English is almost uniform throughout the United States, with some differences based on geography and ethnicity and race and age. If you are an actor trying to imitate accents, I think Texas speech is more difficult than several other accents.

The California accent is probably more neutral. Accents in different parts of the country aren't that strong in general, other than sometimes in the South. In New York or some places which also have strong accents, the accents are strongest in people who don't leave that place very much and live in neighborhoods with groups of people who have been there for many generations. I don't know which accent is hardest to learn, typically the accent that you learn in the beginning is the easiest for you.