Interested in Language
This is Lucas' / Lucas's basketball.
Which word should I use?
I can't imagine a situation in which I wouldn't pronounce the second S. Saying "This is Lucas basketball" would be incorrect. I would accept it written with or without the second S but either way, it would be read aloud as "Lucassiz".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.