In some cases, there are big differences between both.
Anime:Sometimes longer than the manga, even though most are fillers.(Example:Naruto)
Manga:Sometimes longer than the anime...in fact some go beyond the point where the anime ends.(Example:Love Hina, Elfen Lied,Genshiken)
The obvious differences are that Anime has voices, and Manga...you have to read.In most cases I prefer the anime...if I have the time though, I'll read the manga.