It's more secure if you a specific username, as someone would need to know both the username and password to connect to the share. But it depends on how and where you are using it.
Why don't you just experiment and try it. Share it the first way, then go to the other Mac and see if you can access it. If not, try specific username and "everyone". You may have to temporarily add another user to your other Mac.
Either add the username on the other mac, or if you are the user on both macs, just login to the other mac through command K on the finder.
You only need to add sharing to the entire drive if you want to share the entire drive, e.g. if it's a media drive at home.