WAN vs Metro
1) Future BW Expansion : Metro-E BW expansion is as very easy compared to adding leased circuits to the WAN.
2) Service levels : Since Metro-E is a service it has better service levels than a traditional WAN link.
3) Finally Cost: In most cases the BW cost derived per Leased Circuit to that of Metro-E Link is higher in most places.
What may make the choice of WAN more practical is the availability of WAN only and no Metro service in a area.
Plus if the customer is not at all comfortable with a shared infrastructure link like Metro-E.
HTH-Cheers,
Swaroop