Is sending payment data over a VPN more secure than SSL? I want to be able to transmit payment data from one server to another but setting up a VPN is somewhat complex versus an SSL tunnel. Thoughts?

SSL VPNs: Great for basic access but not for power users (http://searchnetworking.techtarget.com/qna/0,289202,sid7_gci834329,00.html) might help you out some.

SSL (if you are using client side certificates) will be secure. VPN offers basically the same pieces however allows you some additional IP level controls but consider the application. IPSec VPNs often require additional overhead to establish the connection.

If the server-to-server connection is going to be persistent, VPN maybe best suited for the application.

You will be using the same or equivalent encryption and authentication algorithms in both cases. A vpn is somewhat more granular and offers you a semi-persistent connection depending on your key lifetime settings. The vpn also offers you more flexibility with respect to additional connections once you have it in place.

As far as security, I would trust both in practical terms.