A lot of our production installs run on VMs running both Linux and Windows. We have used Cisco Unified Computing System server platform as the hardware and VMWare for virtualization. We have tested production infrastructure with load generators and it stood strong and stable for 4k concurrent users. We did not find the breaking point though. Personally I have observed older versions of RHEL were not so VM friendly, but RHEL 6.x works really good. If you have an optimized underlying host architecture, I/O, storage subsystems, performance and stability, VMWare offers best performance.
Apart from this, JVM performance is what really dictates windchill's performance. Solaris is my pick in that regard, i have observed the best performance for JVM in Solaris. Then Linux and Windows.If you can afford to spend on SSD, memory swapping will not take a toll on overall OS and JVM performance(We have it for DB not for App).
As Stacy mentioned above, backup and restore, DR capabilities of VMWare Infrastructure stays unparalleled . It comes very handy when you have a challenging code build and deploy cycle with multiple environments. We have used snapshots a lot in testing some configuration updates and offers a faster way for recovery in the event of failures. We have used Zerto for VM mirroring for DR. VMotion offers high availability protecting you against any underlying hardware system failures.
Thanks
Binesh
Barry Wehmiller International