All I found was
https://social.msdn.microsoft.com/Forums/azure/en-US/f660c4b2-a2dc-4919-81f1-2efd46af8d4c/managed-ews-and-tls-12If you configure EWS to use TLS 1.2 then security for both options should be the same.
Not sure why you're concerned about security during message transport, security at rest is probably more important.
Regards. Al
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