The US chips, the USII and below in particular, are generally a lot slower than even the high end Pentium III chips. Of course, P4's only go in uniprocessor systems, and dual configs for their Xeon line. Sun chips on the other hand, can go into boxes with many, many CPUs, and easily clustered to over 1,000 CPUs.
But, for uniprocessor performance, An Athlon or Pentium 4 will beat out that chip (not the US III, but the US IIe for sure). Of course, the software you run might have a really bad port to x86, if at all. In that case, the US should be faster. Like andy said, it depends on what you run.