First thing that comes to my mind would be any php programs. Don't know which version ubuntu uses, but I'd guess it is two to four minor version ahead of debian. If not even a major version. (major here would be 5.3 to 5.4). Check that your scripts run with the lower version as well.
I don't have any ubuntu server but I'd say they work the same with apache and vhosts. In debian vhosts are in /etc/apache2/sites-enabled (symlinks to ../sites-available).
Pros definetly less upgrades as you said. Less time, less chance of breakage.
Con less upgrades so not as up2date version as ubuntu. Might not be a real con if you don't need shiny new stuff.
Con Lacking behind up2date versions of programs. No shiny new stuff. No new features.
Pro lacking behind up2date versions. Well tested stuff. No(t many) new exploits.
Just some random thoughts. But as ubuntu is based on debian you should not have major issues. But the small ones can take some great deal of time (if any). Just do a test install at home and see whats the outcome. If you come upon some version difference that would make your site break you might want to wait some time for wheezy release. Which should be in April 2013 as far as I know.
Was one minute too late