I’d use the export tool but split the files using a tool like http://rangerpretzel.com/content/view/20/1/
You may need to check everything individually to make sure there’s no conflicts like same name categories, tags and usernames.
Check posts, pages, comments, custom fields, terms, navigation menus and custom posts for potential conflicts.
You may want to set these up first manually and then import one category at a time if there’s any conflicts.
You can also use the bulk edit feature to make changes if needed.
You may also have a problem with same name image file names which you will needed to change manually.
You might also want to change the site address and server path using a plugin like WP Migrate DB or similar which enables you to change links in the export file if you decide to export the database rather than use the export tool in WordPress.