This post has been waiting too long to be answered. This is probably because there are so many viewpoints on it. I found the following links which discuss the topic and hope that they help.
It would appear that Lotus Notes wins overall but I would recommend taking a look at what the users would prefer. Any time I have changed software it is followed up by training which is always followed by personnel asking questions, seeking further guidance, and everyone getting acquainted with the new software themselves.