Debian :: Different Functionality In Aptitude And Apt-get?

From what I understand aptitude and apt-get are both valid package management tools and it should not matter which one you use (of course the user interface is different, but the basic functionality is the same). I found something which makes me believe there are differences: I ran aptitude install gnome-themes-more and it did nothing because the package was already installed. Then I ran apt-get install gnome-themes-more just to see the difference, and it also did not install anything, but it marked the package as manually installed.

The I ran again apt-get install gnome-themes-more and this time the package was not marked as manually install (obviously because it's already marked). This makes me believe that aptitude did not mark my package as manually installed (I would have pasted the command output as well, but I don't have it anymore). So, is the core functionality of those tools the same or not?

I used until now apt-get and wonder if I should have used aptitude. I have found some wikis which recommend the usage of aptitude but I could not find out if this reccomendation is based only on the UI improvements or are there also improvements in the core functionality. I'd like some hints from more seasoned debianers about which one to use, or whether it matters. I'm maily using command line, so user interface is not an issue,