If you only have 6 acres of watershed, you would do better with a .5 acre pond. Your pond builder may be able to increase your watershed area.
Is your land open or wooded? If wooded, it will cost a little more to build.
From what I've learned from building mine, I think I would worry more about building it right and cleaning and leveling things up around it more than how big. I used all my budget trying to get it as big as I could. If you have a very large budget, then go for both.lol!
