in my opinion i think manga then anime the manga has "more" of the story and the anime sometimes has "blanks" and "jumps" in it and if you have read the manga first you'll have a better understanding of the story and etc.

ex: one piece manga started with the story of luffy's childhood and the reasoning for his pirate adventure but in the anime the drop yah right in a fight with fat alvida but dosn't explain luffy's childhood until ep 4 and it also leaves out how he got the scar on his face...