Solaris 2 is more compatible with the rest of the UNIX industry.
Other major UNIX vendors including IBM, HP, SGI, SCO, and others
are based on System V rather than on BSD (though some of them
are on SVR3, not SVR4). All but one commercial PC-based UNIXes
are System V based (and mostly SVR4); the only commercial exception
is from a small but interesting firm called BSDI.
Solaris 2 is where Sun has been putting almost all its development
for the last few years now. There will be no new development on
SunOS4; already much of Sun's add-on software is only available
for Solaris 2. Solaris 2 is the only supported MP OS on all but
the old 4/6x0-1x0 w/ Ross 605 modules. All the UltraSPARC systems
require Solaris 2.x.
Sun software is being released first for Solaris 2.x and usually
no longer for SunOS 4.1.x (No Sun JVM for 4.1.x)
Solaris 2.3 and above feature a standard X11R5 release of The X
Window System, a benefit for those who didn't like NeWS or the
V2/V3 OpenWindows server. (It's still called OpenWindows, but it is
the X11R5 server with Adobe DPS added in). It is as fast or faster
than MIT R5 or XC R6 (depending on the platform) and supports all
Sun graphics hardware.
Solaris 2 is more standards-compliant than Solaris 1/SunOS 4.
Solaris 2.6 is also Y2000 compliant, so upgrading to the latest
Solaris release is a must.