I have AirMail and used Spark. I am keeping AirMail, but tend to mainly live in Apple Mail. Sometimes I have to use Outlook for work but I hate that thing. Their "new" interface still does not work with Enterprise Exchange servers!
Thank God for imap...
I have to deal with multiple accounts - currently 8 of them - and I like the compact interface. "Text, the new frontier!"
AirMail is prettier, but not as condensed for large volumes of mail.
My approach is to read, file the few items I keep in bins, delete, and then archive stuff I might need every couple of weeks.