well, it's not like you have to eat a DF in order to be strong. Haki alone can get you on yonku level. just ask shanks.They're both awesome. Both have awesome features. I'd say Hunter x Hunter, though. In One Piece, you kind of have to get lucky and find a DF. Without it, you don't have as much potential to be a top pirate (People like Mihawk, Zoro, etc. pulled it off). However, in Hunter x Hunter, everyone has Nen and their own special power/class. You don't have to achieve it, just train your Nen.
So, I'd say HxH, but they'd both be awesome to live in.