The dynamic public IP Is going to be a bad time if you're trying to string up an IPSEC tunnel. If it changes, your tunnel will go splat, and you'll have to update your settings with the new IP address.
AutoVPN doesn't care if your IP is static or dynamic, but you can only do AutoVPN if they're within the same organization.
Other thought: You own both of these sites? And they belong to the same business? Is there a _business_ reason why the orgs are separate?
If there's not, I'd call Support and discuss your options for merging orgs. You would have to reconfigure equipment, but if you've got a branch office with an MX, a switch, and an AP or whatever, that's pretty fast. Move branch gear to main site org during outage, re-config, boom. AutoVPN is viable.