I hear brotherhood is a lot darker and keeps closer to the manga. there are a people who died in the original show that don't in brotherhood and some who didin't die in the original actually die in brotherhood. its different.
One piece is good, and the ORIGINAL english voice cast was good (not funimation, sory Black Star, you're no luffy). But the original english voice cast only lasted a season or two before rights were bought by funimation (though Picallo makes a great Zoro). I'd say watch the japanese version online. Voice cast is the best and is the most complete. Its one of my favorite shows honestly. its the perfect blend of action and comedy and all the crew is so well rounded and well made that I can't quite pick a close second favorite character (luffy is the favorite).
However, if I had to go through any more episodes in the desert or the freaking sky island I would of had to kill somebody. Though, its only because they are masters at wasting time (and usually in a good way actually) that they keep from catching up with the writer and cancelling the show. I'M LOOKING AT YOU, REST OF THE ANIME WORLD!!!