I have recently switched to Opera Mail and I think it is the best email client by far. As an entrenched Outlook user the transition was seamless and I love it more and more as I get used to it.
I have tried every mail client out there and nothing compares.
However there is 1 feature in "Mailbird" that is a winner. This is the small button on the edge of the page that allows you to move or tag an email with the folder name. This also shows up at the top of the email.
I know you are using labels and these work really great but when you have thousands of emails that need to be sorted it makes sense to use the folders as this is how they are sorted on the server.
If you can do something like this then "I" believe that you will have the best of the best!!!!