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.