I'm finally convinced of what Sun has been saying for a while now: Solaris is faster than SunOS in most aspects, but slower in many ways because of increased functionality. On the slowest machines, it's so slow it's nearly unusable, making the increased functionality a moot point. It also makes better use of RAM but requires more of it. Some older workstations have maximum RAM configurations that are just too low to make Solaris a reasonable proposition.
SunOS is just too old for most of us. Don't use it unless you have a good reason.
The open-source Unix operating systems that support SPARC hardware--namely, Linux, NetBSD, and OpenBSD--are good alternatives on the older machines. They have lower resource requirements than Solaris and are recent and well supported.
Solaris is a good option for newer machines (SPARCstation 4 and up) equipped with enough RAM (say, at least 64 MB). It is particularly useful for providing robust SMP on the MP-capable desktop machines (SPARCstation 10 and 20 and Ultra 2).