I’ve used a few different software packages through the years. The main thing I’ve learned is that the software doesn’t make you a better writer. People have written some utter crap in Final Draft. Others have written works of genius using mechanical typewriters. Tarantino actually writes his scripts by hand.
I use Final Draft nowadays. That’s only because I thought that I had to use it. If you are working collaboratively in a writers room you might have to confirm to a standard piece of software. But most of the time you should use whatever you feel comfortable with.
For ages I used Celtx. It was free. It was simple. It did everything I needed.
I tried using Scrivener. I was told that it was great for planning your story. Maybe. I didn’t notice any surge in creativity. And I couldn’t get a flow happening.
Save the Cat software. I’m not a cat person. Even trying to use it as a basic tool for documenting my story elements caused me grief. I know some people swear by the STC system, but it’s not for me.
The best tool I’ve found is… Google Docs. Yep. I can use the documents to check together all my notes, and use the spreadsheets to plan out all my beats. I can access it at home or at my day job. I can gather all my thoughts and then use whatever software I need for the actual screenplay.
So, just use whatever you want. If you feel happy with it, if you’re comfortable using it, and it doesn’t impinge your creative flow… just go for it.
One quick note. Learn the keyboard shortcuts for the tool you use. Reaching for the mouse can break you flow.