Manga: It's more detailed when comes to the plot and doesn't contain fillers (usually). It always concentrates on the main parts of the story. Through reading from the first chapter to the latest you can see that the author's style is improving and is more beautiful. After many released chapters authors can decide to allow to make an anime based on the story. This usually happens when manga is getting more fans, but not always as some of my mangas that I read hasn't released an anime even though they have lots of chapters.
|Example 1: Lizzy in Manga is alot different then in the anime & more detail on the stories|
|Example 2: Improvement in Art Style|
Anime: The best thing is that you can see your characters moving and see the action which is not in manga. In addition, it is colourful, has background music and the most important thing for me an Opening. I wish every time I read a new chapter of my favourite manga I would see the opening XD. Yet, that's not how it works. However worse drawbacks are the fillers or missed parts of the story which haven't been included in anime but only in manga. In other words less detail in the plot.
I only read manga if the plot is good as I'm the laziest person as I cannot force myself to read >_< So I'm very picky when comes to pick something to read, yet I watch whatever anime people say is good :/ In my opinion, I think that you should do both. If you liked the anime, you will like the manga! However, it may not go the other way as sometimes anime are shitty whereas manga is really good :)
If you haven't toke part in survey please let me your opinion in the comment; What do you prefer and WHY? :)