implicated differences between は and が


I understand a lot of Japanese is not spoken and implicated through context,
but don't really understand the culture behind it. What are the implicated
differences between は and が in the following examples?

食べ物がある？ // 食べ物はある？

or

トイレがある？ // トイレはある？

Is one more common, or more polite, or more usual, or offensive to say than
the other?

