It's really not possible to make a blanket judgement. The lot of high end systems boil down to brand name, software compatibility, "one throat to choke", and vertical integration. While the first two are mostly meaningless, the last two can have profound network effects. IBM has been able to realize nice vertical integration across most of their current systems, and they have some world class people doing CPUs, compilers, OSes, application stacks. If you have a problem with the mainframe you have one company to deal with.. that could be very good or very bad (somewhere brand name aka reputation does matter). In consumer terms, it's kind of like Mac vs PC.

It's also not so easy to judge x86 as low end. Stratus and HP make some really high end fault tolerant/instruction retry x86 servers that could be considered mainframes IMHO.

Low end x86 servers are "good enough" for a large portion of what people use servers for, and a massive amount of work has gone into making distributed systems that can scale and handle faults semi-predictably. That said, I'd much prefer my bank account to reside on an IBM mainframe SysPlex for the time being.

Sun kind of struggled in the CPU market as well as indecision around x86 for a long time, it's one of the primary reasons they are now part of Oracle. Those E3500/6500 systems are very high quality and would probably still be running fine today if performance isn't the only measure of worth.

I've long been a fan of IBM's POWER system p or whatever they want to call it depending on the day of the week. It's positioned somewhat in the middle of x86 and mainframe, but you generally get more powerful systems than x86 with mainframe inspired reliability. Still, these fill a much narrower niche than cheap x86 boxes.