What is the main difference between SAT and ACT, if i’m getting a good score in ACT, should I get a high score in SAT too? is the math in SAT harder then ACT or grammar or something?

The general consensus is that it really doesn’t matter which one you take, if you get a good score in one of them, that should be enough. Look at ACT and SAT score equivalent tables such as this one

Personally, I took the OLD SAT and I struggled with the english parts (I’m a science/math student) but luckily, after working hard and getting a bit lucky, I ended up getting a 2300+ on my first sitting. I never thought about SAT vs ACT, I just wanted to get it done and over with and just stuck to one. Don’t waste time and start cranking cause at the end of the day, the difference is quite small.

From what I’ve heard from friends, the new SAT is much more easier than the old SAT and also more similar to the ACT than the old SAT was, but I have heard that scaling is quite brutal (less margin for error)
The ACT has been around for longer and theres more resources/pastpapers so I would recommend you stick to the ACT, especially if you’re getting a good score already.