It is recommended you review the release notes for Async OS releases to make your choice of upgrade.
www.cisco.com/c/en/us/support/security/email-security-appliance/products-release-notes-list.html
From the release notes general deployment releases are considered stable to use while maintenance deployment contain critical bug fixes and hot patches.
Do note that x70 appliances including C370 require 8GB of RAM installed in order to upgrade beyond Async OS 8.5 as per below field notice.
https://www.cisco.com/c/en/us/support/docs/field-notices/638/fn63931.html
Regards,
Libin Varghese
