Hi,
OpenVPN supports a big diversity of different protocols/addresses/ports and operates in Layer 3 on IPfire (TUN) whereby SSH tunneling forwards only one protocol/address/port and operates on Layer 4. OpenVPN produces therefore much more overhead as SSH does. Also, OpenVPN on IPFire has been configured that mostly all clients does have a working connection after the first setup. If you have several clients, you will need to have the weakest connection settings (Transfer Unit) as basis which slows down other may faster connections, so everyone needs to adapt it to his specific needs.