It really depends on the application. Having some insight on the telecommunications sector, I would guess you’ll more than likely be connecting to a provider using 10G LAN-PHY on a sufficiently modernized network, or OC192(US/Asia) or STM64 (Europe). WAN-PHY connectivity is rare and actually requires more bandwidth on the telecom provider’s backbone, and SONET/SDH is being phased out of high-bandwidth connections. I think you would mostly likely end up using the ER XFP, but you MUST check with your provider to see what protocol they’re going to be handing you at the demarcation, or make the demand for LAN-PHY 10G yourself so they can card out their equipment accordingly. If you are running over leased dark fiber, it’s really up to you, and you’ll need to read up on the pros and cons of WAN-PHY versus LAN-PHY, which I’ll leave to Google.

From the telecom provider’s perspective, this what the layout of a typically 10G circuit looks like:

Your 10G XFP—-Their card’s 10G LAN-PHY interface::::Their card’s OTU-2(or 3 on muxponders) interface—-[OTN cloud]—-OTU-2(or 3) interface::::10G LAN-PHY interface—-10G XFP

:::: = electrical connection within transponder/muxponder card
—- = fiber optic cable
OTU-2 = G.709 10G protocol used within the provider’s network.
OTN = optical transport network