Hi Doug,
I understand your concern, and I agree that Airmail seems to have a better support, community and frequent updates.
I'm personally looking to a substitute till Apple fixes Mail, then I'll go back to that.
The thing I don't like about Airmail, is the interface, and the filtering system, which in my case (and it's just a personal preference) slowes down my daily work routine.
I liked Postbox more just because it looks like Mail, and more or less has the same features. Also I appreciated the fact that it imported all my email accounts and settings from Mail automatically, which means it was ready to go right away.
On a long term, probably Airmail, is going to be more stable (updates and support), and if you have just one or 2 email accounts will be more than enough.
If you prefer a more essential design, using multiple accounts, with several subfolders, high usage of filters and labels, then I think Postbox is the winner.
Patrix