Welcome to the DIS, CaptainNemo!
Both the Poly and the Yacht and Beach Club are fantastic resorts, each with their own pros and cons. Which resort will work out best for you really depends on your family, and what you do during your trip.
If you go to Epcot and MGM alot, the Yacht or Beach Club would be better. You can walk or take a boat to both of these parks from the Yacht & Beach Club.
If you have young kids, and are planning on spending lots of time at the Magic Kingdom, the Polynesian would be a better choice. It's a quick monorail or boat ride to the Magic Kingdom. You can also walk to the Epcot monorail at the Ticket & Transportation Center, so the trip to Epcot can be pretty easy as well.
The pool at the Yacht & Beach Club is really something else. It has an awesome slide, a lazy river section, and is beautifully landscaped.
The pool at the Poly is nice, too. It's a zero entry pool, and the beach is steps away.
In my opinion, the Poly has better dining options inside the resort, but the Yacht & Beach Club has more dining options within walking distance of the resort.
I think you'd be happy at either resort. I'd suggest figuring out which parks you are most likely to visit the most, and then decide based on the location.
Karen