You need a -z to activate compression. But, nonetheless, please always use rsync when copying host to host.
I'm not aware of any attributes that tar doesn't preserve.
Also, I find that if I'm going to copy the data once, I'm often going to copy it twice, or which to get a more up to date version of it at a later time. Rsync clearly wins in these cases.
Finally, from the compress flag on rsync: Note that this option typically achieves better compression ratios than can be achieved by using a compressing remote shell or a compressing transport because it takes advantage of the implicit information in the matching data blocks that are not explicitly sent over the connection.
Remember: Rsync trades CPU and disk i/o (lots of disk i/o) for network bandwidth.
In the pathological case "thousands of tiny files over a fast network" it can easily be orders of magnitude slower than a straight tar.