On Ubuntu, why "sudo apt-get" sometimes and "sudo aptitude" other times?

"apt-get and aptitude are both frontends to dpkg. Use one or the other but be consistent. Aptitude is newer and is suppose to be easier to use. It also unifies some of the apt-* functions. You can use aptitude to search and install while with apt-* you need apt-get and apt-cache for installation and searching respectively.

I usually replace all apt-get examples with aptitude when copying & pasting commands.

Run aptitude without any options to get an interface for selecting packages. I don't ever use it so I don't know how friendly it is."

"Long-time Debian users have told me elsewhere that aptitude has it's own database that it maintains alongside the one that apt keeps. This means that using both interchangabley will result in some confusing things happening sometimes, particularly in aptitude when it seems to have a slightly stale copy of the package state. I have seen this happen: it results in things like packages you mysteriously can't remove or can't install.

The GUI program synaptic doesn't have this problem because it relies entirely on the apt database."