It would be exactly the same.
OWA and ActiveSync both use HTTPS as the transport. If you think you know better than cryptographers regarding X.509 and SSL, then by all means, don't use it.
From a cryptographic standpoint, a BlackBerry is just as "secure" to a mobile phone using ActiveSync.
There are some organizations that choose not to have Exchange touch the Internet. I think this is wise. These organizations use a more robust inbound MTA which shuttles mail back to the Exchange server. This prohibits to use of OWA et al unless you're also supporting VPNs or similar.
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