The Net::OpenSSH documentation has a section describing the pros and cons of each. Here are some excerpts related to the ones you are asking about, but the documentation lists more:
Net::SSH is just a wrapper around any
SSH binary commands available on the
machine. It can be very slow as they
establish a new SSH connection for
every operation performed.
Net::SSH2 is much better than
Net::SSH::Perl, but not completely
stable yet. It can be very difficult
to install on some specific operative
systems and its API is also limited,
in the same way as Net::SSH::Perl.
Net::OpenSSH has a very perlish
interface. Most operations are
performed in a fashion very similar to
that of the Perl builtins and common
modules (i.e. IPC::Open2).
However, choice of platform may limit your options.
On the other hand, Net::OpenSSH does
not work on Windows, not even under
Cygwin.
Granted, the list is biased towards Net::OpenSSH, but it gives you an idea of the major differences between the modules.