1. If you have access (ie. hosting their site), then just make a test instance where you transfer all of their data to Revo; Showing them the significant upgrades and changes.
Although Evo is still being developed, much of the core team is working on Revo, which is a major selling point. If they're worried about security, having direct access to core team security patches is the way to go.
The first mistake they're making is assuming that there is any one CMS thats free from security issues, so switching will not benefit them in the longrun.
From my personal experience, the MODX community/devs are much quicker in releasing patches to security issues. You might go weeks without a drupal/wordpress/etc security patch, or gain 0 support from their community when the security breach is caused by one of the 8 trillion plugins that have been left for dead.
Many of the more popularly used packages are either heavily supported by or directly maintained by the core team, which is more than enough when you want to install them.
So my list is:
1. Many popular working packages are also updated by the core team/expert community members.
2. Security patches for Revo usually go out within a couple days maximum after being discovered vs weeks with 'more popular CMS's, whom might leave a note to disable some integral feature on your site rather than patching it immediately to avoid releasing a patch version earlier than scheduled.
3. Opting for another CMS doesn't inherently deal with security issues. (These could simply be caused by a flaw in PHP itself that wasn't documented or another edge issue)
4. Create a development copy of their website and make a video (so you dont have to do it live in front of them) showing them the upgrade process. It allows you to iron out any kinks ahead of time.
5. Not sure about what you're doing now, and I know its a hassle, but make sure that YOU keep your client updated. We're regularly updating 50+ sites every time a new version comes out for our clients security, (since thats generally when new versions are released; early for a security patch). I think the key thing is how YOU handle security moreso than how modx does. (this isn't an attack

Just noting)
If you notice anything strange on a website, i'd report it IMMEDIATELY. We had a russion spammer changing values of our text fields on a couple sites that we manually patched before MODX did, and before we could write a patch/notice post in the forums, they had already fixed it and issued and update.