Both AT&T and Verizon are committed to leapfrogging from 40G to 100G as soon as possible, according to a recent report by Telecom Pragmatics; however, AT&T currently appears to be more willing to go with a 4x10G and a 10x10G muxponder approach. Verizon seems more interested in serial 40G or 100G deployment.
Given the overhead that needs to be carried, Verizon sees combining multiple 10Gs as untidy, as well as being more difficult for the central office technologist to manage and maintain. AT&T evidently likes the idea of taking advantage of the spectral efficiencies and the lower cost of the 10G components. The 10G rate sub-multiplexer is also a lot more usable because there are interfaces at that rate everywhere in the network.
AT&T has also been the most proactive large carrier in addressing short-term capacity needs beyond just dealing with 40G interfaces on routers. Verizon is focusing more on the long term by provisioning any 40G network for future usage at 100G.