I want to start Monster. Should I read the manga or watch the anime?

For example, I would rather read One Piece manga (bad anime pacing) and watch Hunter x Hunter anime (horrible manga art).

I would suggest manga.

[img]http://media.tumblr.com/e43f35cfdd762cc666571a6dd0f163d5/tumblr_inline_mhbel8Cy8V1qz4rgp.gif[/img]

Choose whichever one you will actually finish the whole thing. But remember the manga is the one that the author created in its entirety, so it's a little bit more "fidel".