The disk is fixed and you would have to upgrade packages afterwards anyway.
Upgrading via the internet will give you the latest packages without needing
to upgrade. If bandwidth is an issue then the disk method will save
bandwidth since a post installation upgrade will not replace everything. If
you have a fast connection and no bandwidth caps there should be little
difference in time because disks are slow. Another issue is that you cannot
upgrade via CD previous to 11.04. You can only do a fresh installation, I
believe (I have never tried it anyway). I am not an advocate of upgrades and
always choose a fresh installation. That means using the CD and starting
from scratch. However, I have a separate home partition and it never changes
which means that I preserve data and settings when I upgrade. I also save a
list of installed packages and a backup of my sources list. Then after I
re-install Synaptic puts everything back in order for me. I have used this
method for the past six years and done a re-installation at least 30 times
without a hitch.