* סידני (Sidni): This is a common and straightforward transliteration, prioritizing pronunciation over literal letter-to-letter equivalence. It's likely the most widely understood option.
* סִּידְנִי (Sidni): This adds vowel points (nikkud) to the previous spelling, making the pronunciation clearer for Hebrew speakers unfamiliar with the name. The specific vowels used might vary slightly depending on pronunciation preferences.
Essentially, סידני (Sidni) is the most practical and commonly used spelling.