Rapid Prototyping gets my vote..I'm in the process of working on a prototype for a new web service that's attempting to make easier a very complicated industry

Rapid Prototyping gets my vote..I'm in the process of working on a prototype for a new web service that's attempting to make easier a very complicated industry (where things are evolving at a rapid pace). I've noticed that we never have any time to create test user cases, however I am working with 2 others who hire outsiders to perform real user tests, which works well enough for the time being...but testing only a few users, with a minimal amount of requests doesn't really give you a good indication of whether or not your application can handle the load of multiple users. From a functionality standpoint user testing is good, but from a stress-testing go-live standpoint it's not really that good an indication as to where you stand