As this is an IPS that you'll deploy in-line with a network segment, you'll be especially interested in MTBF and fail-open capabilities as any maintenance downtime or fail-closed condition will very likely block all traffic on that segment. Traffic volume rating is important to know so you can determine if the number of non-analyzed packets will require the deployment of additional down-stream sensors. The ability to utilize a network TAP isn't important, as an IPS is deployed in-line to your network and must have the capability to discard packets as dictated by policy.