As far as I know, iAS6.0 MAY not compare *Very good* with other appServers. Though it had many cool things, it also has many other problems and unfinished tasks in it. This might be due to various transitions and evolution. This is my personal exp.
With S1AS, since its not yet released, we may not be able to compare with other competitive products. Sun has itself to blame if it has raised expectations on S1AS cos the whole community is watching for its release and for evaluation.
If you are interested you may look at Dynamo Vs iPlanet at
http://www.ugcs.caltech.edu/~werdna/nnh/technology/Dynamo-iPlanetMatrix.pdf PS: Dynamo is one the first of AppServers that introduced many features long-ago that some dont have even today.I'd compare it to Apple/Xerox in PC world.
Raj