I have limited internet and was wondering if it uses less bandwidth to upgrade to 12.04 through update manager or if it is the same dowload size as an ISO image downloaded from the website. Which would use the least amount of internet?
If you want it to be faster (more speed and less time) , do this
Install apt-fast from http://www.webupd8.org/2010/01/new-apt-fast-version-now-with-full-full.html
and then run the command
Download size will be the same in this case but it will be faster because of multiple internet connections.
If you are still getting lesser download speeds, I recommend downloading the 12.04 CD and upgrading from it (backup required data in case something goes wrong)
The difference between two installations is
1)Update manager will update all the packages installed on your system to newer versions if available in 12.04
2)CD upgrade will try to update only some packages of 11.10 to match default packages of 12.04 and if not possible might update some dependencies as well but tries to be minimal.
So obviously, 2nd one is more faster
Download size when using Update Manager depends on the number of applications you have installed on your machine - generally, I never saw the number below 700Mb and sometimes it was as high as more than 2Gb, so if all you need is a running Ubuntu 12.04 in any shape or form then a clean install would use less bandwidth.
However, if you're going to make a clean install and then re-install all applications you have currently installed (and it's likely you'll need to install updates to be able to do that too) - then the difference would be much less. Or maybe even the second method would take more bandwidth.
So, the final answer is "it's about the same, plus-minus a few Mb" :)
Cold hard numbers: I installed Kubuntu 11.10 in VirtualBox then tried to upgrade it to 12.04 - the upgrade tool said it needs to download 614 Mb. The ISO size was 701 Mb. So we may assume that upgrading requires slightly less bandwidth (<100 Mb difference) than downloading the ISO and then re-installing all the software.