duty finder is good for when you need to just jump right into a dungeon and do it. Party finder is good if you need to specify anything for your party before doing a dungeon. For example you are new to a dungeon, so if you make a party in party finder, you could put in the comments that it's a learning run or something like that. This will increase your chances of getting new people but also people who don't mind helping people new to the dungeon.