Hello Matt,
all vendors usually count twice the speed of their products (tx+rx) this is done also by Cisco.
Some may count tx and rx packets at the external ports and tx and rx packets towards the backplane/switching fabric.
see this document on network world about "vendor math"
http://www.networkworld.com/community/node/45289
from a technical point of view whar counts is to have a non blocking switching fabric.
Also there are other aspects to be considered:
QoS toolset, support of multicast traffic and so on.
Hope to help
Giuseppe