Final Draft offers a free trial - maybe give it a go! Having used both, there's really no reason why you can't write in CeltX! But, I am a big fan of Final Draft myself. It's just an all around solid tool - but, of course, doesn't make anyone a better writer. I do think there should be something said for ease of use and knowing that something is going to work when you need it to - you are able to free up your mind to focus on the creative. I honed in my skills on free software and didn't make the plunge into buying my own license of Final Draft until I was working regularly with industry writers. It's true what they say - it's industry standard. If people are seeing your scripts in PDF form only then they will be none the wiser! Assuming your formatting is up to par. However, if you will be sending your working files (program files) to other writers then it's definitely worth picking up Final Draft. A good rule-of-thumb to ask yourself is "Am I making money with this?" and if it's paying for itself then go for it! If not, maybe keep at it with free software and invest in something else that might help creatively? All that to say, I love Final Draft and you probably would too if you do end up going that route. Keep up the writing! All the best in your creative process!