That choice depends on your own site and what's most convenient for you.
You can rewrite either one from
but by default, anything without an extension will send your server looking for a directory and then for the index.whatever file in that directory. If they don't find it, some servers will then proceed to Option B, looking for a file with that name. It depends on whether a particular mod is present and enabled, but to be safe you'd better spend some time with your .htaccess.
Never link to a named index page as a URL. It's Duplicate Content. Link to the canonical URL (ending with a trailing slash) instead. It also causes problems when you change the technology that runs the site.