fail to see the difference. It’s people posting and people replying just like here. Or do I need to actually sign up to see the differences?

Some of the differences are justthings an instance an admin can change on Mastodon, such as

• Post character limit is 4,000
• Search results include post contents

But there are things that would require a Mastodon admin to to fork and modify, too

• Quote boosting is supported
• You can format your post with markup
• Emoji reactions instead of only 'like'
• Antennas are a more dynamic way to group posts than just following hashtags. You can specify keywords to match from the post body (both include and exclude), specific users, specific servers, include or exclude reply posts

It's ultimately just another front-end to ActivityPub and the Fediverse. Use whichever one suits your preferences as they can see and follow each other regardless.