Honestly, if you try to point to a single number and use that as justification for making a value judgment, you are doing yourself and your stated purpose a disservice. You want to look at everything. You want to MEASURE everything. And you need to be able to tell the story of what that particular measurement means. For example:
* number of requests per second that can be serviced at a particular resource utilization level - say, 75% of peak. If one server can process twice as many in the same time period at the same resource level, clearly, it can scale higher
* ability of the server to recover after being maxed out for an extended period
* error rates while at peak load
* ability of the technology to scale by adding hardware resources, such as in the cloud - not every technology can