I've heard very good reports on Xen from some who are using it. I
never heard of Solaris Zones until now, but my first thought is, why
lock yourself into Solaris?
Solaris Zones sounds like Linux V-Server or the FreeBSD Jail facility.
These are all inherently different than Xen. With Xen you can run
multiple different kernels, even entirely different operating systems
(as long as they support Xen). The new thing about Xen, is that
unlike VMware, it can do this efficiently (like IBM's VM) on commodity
processors (unlike IBM's VM).
See also these old threads:
2004-02,
2003-06,
2003-02.
The problem with Debian, currently, is that there is no support for
x86-64 (neither AMD nor Intel) in Sarge. So either you run 32 bit x86
Debian, or you use the unofficial (and "unstable")
Debian AMD64 port.
Neither of which is a particularly good choice for people buying
x86-64 servers now...
