If you intend to install ROS on bare metal, then you better wait for ROS v7. It will come with greatly improved support for modern NICs. If you intend to run CHR, then HW support is up to Hypervisor you're going to use.
In any case I'd go for CPU with smaller number of high-performance CPU cores ... with HyperThreading disabled. Number of CPU cores help with huge number of parallel IP streams, faster cores help to achieve better single connection performance and some other tasks (such as BGP convergence). If you'll go for a new server, then you won't be able to get anything less than 6-8 CPU cores which would already make pretty powerful router.