Which native server is best, in your opinion, to implement long-polling / Comet? The first target application is chat, but there will be other implementations - we basically
Try netty-socketio project. It's a Java framework with long-pooling and websocket support using famous Socket.IO protocol.
I don't think there's a significant difference in this regard.
IIS + WebSync is a very straight-forward, scalable and extensible solution for server push. There is a free Community edition I highly recommend checking out.
I just saw this blogpost from Scott Hanselman yesterday. It looks very promising.
http://www.hanselman.com/blog/AsynchronousScalableWebApplicationsWithRealtimePersistentLongrunningConnectionsWithSignalR.aspx
It features an opensource product called SignalR which is available through nuget.
You can find an example chat application in the sources @ github
https://github.com/SignalR/SignalR
I know that special attention was paid to Comet support in the Grizzly engine used by Glassfish. It wasn't treated as an afterthought.
Both Java and .NET platforms have enough capabilities to handle your needs. If you choose Java : You may start with DWR otherwise, on the .net side PokeIn library should be the choice.