>Outlook is better with calendar and scheduling, but I prefer to be able to say NO myself without somebody pointing out that I have not scheduled that time slot ;-)))

Not to be taken that Thunerbird doesn't have calendar or scheduling - it even detects when there's one of standard invitations in an email and you respond with a confirmation. It creates a slot in your calendar with the description taken from email's header.

I guess you think Outlook has more bells and whistles in that area, which is quite possible. I think both calendar and tasklist were created as add-ons by contributors, then included into the official build. Both are neat and simple - actually, the tasklist could have a few options more, that I remember existed in an alternative add-on few years ago.