Hello,
It's hard to tell without knowing your requirements. It depends on the geographical distance between the Main Office and the Branch Office, on the availability of a particular service with your service provider, price and other options.
One distinction is obviously the provided speed of the interconnection. Another difference is the layer on which the service is provided - the Metro Ethernet usually provides a Layer2 service while the MPLS VPN is very often a Layer3 service. With Metro Ethernet, you can create a virtual Ethernet trunk between the Main Office and the Branch office. Also, the Metro Ethernet can allow you to transport virtually any Layer3 protocol over the virtual trunk - IPv4, IPv6 or other payload. MPLS L3 VPNs are routed so you will set up a routed IP connectivity with your provider on both sites. The particular routed protocol will have to be negotiated with your ISP - right now, the MPLS VPNs support IPv4 and IPv6 (the IPv6 is supported using a IPv6-over-MPLS technique as there is no native support for IPv6 MPLS on Cisco devices at the present time).
But really, these are just selected technical characteristics of different VPN technologies - they could be described on and on but I don't know if that tackles your point. Perhaps if you summarize your requirements on the service it would narrow down the choices.
Feel free to ask further.
Best regards,
Peter
