If you're going to only do one read the manga but if you have time I recommend both.
The great thing about the anime is that you get to see it all in color, you get to hear their voices, and the fights are dynamic. The bad thing is the copious use of flashbacks, the filler, and when they stretch things out for just too damn long.
The downside of the manga is that I can't find volumes that don't have pretty crappy scans (I'm up to 23 right now). This site has decent text translations, though: http://www.mangascreener.com/stephen/onepiece/onepiece.html