They both have fairly major loopholes that can be exploited. You can move your site around week from week, so the banned list is always out of date for example. Content filtering is just generally hopeless. This system is so easy to avoid it is laughable. They could for example, use encrypted web pages and then the filtering server will never know what is in the page.
Encryption is built into every web browser right now.
Nothing to download, nothing to change. If i connect on port 25 I want to send mail. But, if they tell their web server to listen on port 25 instead of port 80, then the proxy filter wont even look at the web pages because it will assume you're sending mail. No checking at all. If they ban sites, it will be reasonably manageable.