Lightning vs Sunbird

More or less. Lightning is just an extension for Thunderbird, and therefore it isn't a standalone application, whereas Sunbird is. And yeah, Sunbird is more full-featured than Lightning, but if you aren't doing much with a calendar Lightning should suffice. Of course, if you don't already use Thunderbird, there really is no reason to make the switch from Sunbird to lightning.

For instance, my fiancee and I have a Google calendar that we share, I already use Thunderbird, so I just installed Lightning and Provider for Google calendar. Since she uses Yahoo mail, and doesn't use Thunderbird, I just installed Sunbird with Provider for Google calendar and voila. It works

Just depends on what your needs are and what you are already using.

I have not used much Sunbird. However, I haven't found any real difference between both except for contact integration.

One real advantage of Lightning is that you may invite "participants" to a meeting you schedule. You may type your own names, but you may also get them from your Thunderbird Address Book. In that case, it is possible to send them e-mail notifications (and maybe feedback afterwards).