Edit: Ignore this, I was incorrect.
Unfortunately, the documentation on -X and -Y is awfully confusing. On a casual read, it looks like -Y is less safe, since practically the only thing the docs for -Y say is that forwarded connections are "not subjected to X11 SECURITY extension controls"...
-X Enables X11 forwarding. This can also be specified on a per-host
basis in a configuration file.
X11 forwarding should be enabled with caution. Users with the
ability to bypass file permissions on the remote host (for the
user's X authorization database) can access the local X11 display
through the forwarded connection. An attacker may then be able
to perform activities such as keystroke monitoring.
So, it's not documented as being proof against hostile parties with root at the remote end; in fact, it's documented as being vulnerable...[1] http://www.openbsd.org/cgi-bin/man.cgi?query=ssh&sektion...
For this reason, X11 forwarding is subjected to X11
SECURITY extension restrictions by default. Please
refer to the ssh -Y option and the ForwardX11Trusted
directive in ssh_config(5) for more information.