Assuming you're using the right settings (such as the iPhone's default setup for hotmail accounts), the the Mail app will use TLS encryption, which is just as secure as SSL in Safari. And realistically, with a hotmail account that's as secure as you're going to get even on a Blackberry.
One thing GMail does have over Hotmail is Google Authenticator, and 2-step verification. Activating this requires you to use a security token (which is an app you download on your iPhone or Android device) to login to web-based Google services. This way, even if your password is compromised, a hacker would still need physical access to your iPhone to launch the Authenticator app and get access to your account.
Of course, this doesn't work so well with the Mail app. But, you can set "application-specific" passwords for your account through Google, that can be activate and deactivated as needed for added security without having to use the token.