OVS plugin will just set up a static network, where it alternatively sets up static L2 tunnels. The control-plane and data-plane is not separated.

Ryu is an SDN solution, where the control-plane is Ryu. It will dynamically create flow entries at runtime.

You should watch some presentations on SDN; Midokura had a great one at OpenStack Summit 2013 entitled Scaling in the Cloud: The Hype and Happenings of Software Defined Networking that presents a somewhat balanced view on the various solutions.

So, basically, it depends on the scale of your platform. Implementing an SDN solution might make things more complicated, but to achieve scale it may be the only viable solution.