ARIA-tags are used for disabled visitors of your site. It’s very nice of Bootstrap, that they support it by default.
Accessible Rich Internet Applications (ARIA) defines ways to make Web
content and Web applications (especially those developed with Ajax and
JavaScript) more accessible to people with disabilities. For example,
ARIA enables accessible navigation landmarks, JavaScript widgets, form
hints and error messages, live content updates, and more.
Source: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA
To answer your question, which one you should use, use only the title-attribute. Because this attribute is used if your mouse go over the button and shows the text of title as a tooltip. With aria-label is not supported in this way.