Looking at this site much of the discussion seems to be about transcation throughput. Am I wrong in thinking that the ability to support x concurrent sessions at the same time is actually the determining factor in determining hardware requirements for supporting client channels - I would be interseted to hear about experiencing in sizing up for supporting such functions as internet banking where the number of transcations may be quite low but the number of concurrent sessions high.
There is no 1 performance criterion for it. Transactional throughput with a close to linear response time is what most people look at. The number of concurrent sessions is usually limited by heap size (as it's memory dependent), so it's not in the forefront as much.