The Sparc 5s were still being made after they dropped the Sparc 20s (apparently due to a government contract), and, as such, their processors were faster at the top end. However, you can put up to, I believe, four processors in a Sparc20, if you can find them, so it can be quite a bit faster.

But the thing is, what kind of price are you getting on a Sparc 5 that you think is so remarkable? Any UltraSparc machine is going to be faster by a long shot (even the pokey Ultra 5s and 10s), and they can be had for a few hundred dollars. A good price on a Sparc 5, IMO, would be around $30.

I wouldn't bother running Solaris on anything other than an UltraSparc, except, maybe a multi-processor 20. But I don't think I'd go with Linux, either. Check out OpenBSD or NetBSD.