On the high end, I'd use Xen, like all the major large-scale hosting providers do. On the low end, I wouldn't bother trying to do fine grained resource control.

It's not exactly insightful to point out my bias against Zones in a response to a comment where I basically say "I'm biased against Zones". I am indeed biased. And I can defend my argument that, as a technological approach, trying to compartmentalize all the kernel namespaces and building ad hoc group-level rlimits is a dead end.

For what it's worth, just as your bias in favor of Zones (which I don't fault you for) is based on professional experience, so too is my bias against them.