I've been a Solaris administrator for about ten years, and I have no idea what you're talking about when you refer to XWin, unless you're just talking about XWindows, which is not a Solaris-specific product; in fact, Sun was probably the latest Unix conversion to XWindows -- they were using NeWS up until about 1990.

Regardless, XWindows is the default GUI for pretty much every Unix out there, including Linux.