It depends how many of you there are and when you're going. If we're staying at Disney we tend to get the Universal 2-park bonus ticket from Atraction Tickets Direct : http://www.attraction-tickets-direct.co.uk/universal-orlando-resort/universal-2-park-bonus-ticket which allows us to come and go for the length of our stay. Sometimes it can make sense to get a room at the cheapest on-site at Uni (Royal Pacific) for 1 or 2 nights because of the included Express Pass (which is a lifesaver at busy times). If there's four of you in the room it works out a great way to see everything and skip the lines as oppose to purchasing Express passes at the park.
We are planning something similar as a family of 5. I would like to stay for at least one night, perhaps two. I can't imagine wanting to drag myself away from WDW for longer than that, as I also want to visit Seaworld. Seriously considering longer than 14 days...
We stayed at the Poly last year for 2 weeks - it was fantastic but at times it felt a little rushed trying to do as much as possible. The girls loved Aquatica but only managed to go once and we totally missed out on Discovery Cove. Hopefully by going for 21 days this year we will be able to relax and enjoy it more and try more things.