If I can send all of my traffic over SSH, is there a benefit to using VPN?
Another words, is VPN more secure than SSH?
VPNs, being designed to tunnel traffic, are in general better at doing so (read: faster). This is by no means a hard-and-fast rule, and I have seen SSH tunnels do just as well or even better than some VPNs.
That said, SSH tunnels are generally easier to set up than VPNs (again, in general -- YMMV), although are oftentimes harder to use (and again, YMMV).
Bottom line: If you're already using an SSH tunnel, you probably will not see a benefit to switching over to a VPN. As always, though, YMMV, and the only way to know for sure if you will get a benefit is to try it out.
As for security, no, a VPN is no more secure (provided that all of your traffic, including DNS queries and other potential "leaks", are indeed going through the SSH tunnel), but neither is it any less secure (unless it's a really bad product, but the same could be said for SSH clients and servers as well).
SSH does port forwarding only. Traffic going out on one port from an interface in your system travels over the SSH tunnel and vice versa.
VPNs that install a virtual networking adapter are more versatile in configuration. For example, through judicious routing rules, you can make traffic to certain IPs only go through the VPN whereas all other traffic won't do that - without modifying the application's configuration.
SSL VPNs make your traffic look like HTTPS traffic from an interceptor's point of view. SSH traffic looks like SSH traffic from an interceptor's point of view. Application-level filters/firewalls might pick up on this difference.