Since you have placed Layer3 devices at each end of the Metro Ethernet interconnection, the fact that this is a Metro Ethernet (which has its own means of stretching VLANs between locations, either using 802.1ad Q-in-Q or 802.1ah MAC-in-MAC) is no longer important because your VLANs are terminated on the Layer3 devices - the ASR routers.
Personally, if I had a choice, I would go with OTV. This is a technology that was designed with the particular goal of accomplishing exactly what you are seeking for - selectively extend VLANs between distant data centers. As opposed to L2TP, it gives you a better flexibility in selecting which VLANs you want to stretch, it allows you to do VLAN rewriting/mapping if necessary, provides STP domain isolation (each OTV "island" is its own STP domain; BPDUs are not carried across), supports redundancy if you have multiple CE routers at the same location acting as OTV edge devices, and avoids unknown unicast flooding. Assuming your ASR devices support it (which they should), OTV would be, for me, the natural choice. What needs to be taken into account, though, is the fact that the OTV encapsulation overhead is 42 bytes, and so the transport network has to accomodate jumbo frames - there is no fragmentation supported.
My two cents. I hope other friends here will share their own opinions! As usual, there is no single perfect answer... :)