For my 50th last year, I stayed at the Contemporary with a Theme Park view. For the DIS 20th event earlier this summer, I stayed at The Boardwalk.
Both of these resorts are great. Each of them have reasons why I would stay there again. The primary reason being the location and walking distance to the parks. In the future, if I plan on spending more time at MK, then I would definitely consider Contemporary again. The walk is so easy and quick. Now if I was planning to spend more time at Epcot or DHS, then I would select the Boardwalk (or possibly one of the other Epcot resorts right off of International Gateway entrance).
So I would recommend you look at what parks you plan on spending the most time with. Then pick a resort that is close. This will reduce some of the travel time to/from the parks. Now, I will say that when I stayed at the Boardwalk, the bus transportation was not great when trying to get anywhere. My experience at the Contemporary was much better. But that could have just been what happened the week I was at the Boardwalk.
I have also stayed at the Swan, which I thought was nice. So the Swan and Dolphin are good alternatives - a Deluxe resort but at more of a moderate price. But if you need Magical Express or the Dining Plan, then those are not available when staying at the Swan or Dolphin.
Besides looking at the location, I would look at the pictures of the room available online. Then get a feeling for what the room looks like. That is how I finally landed on the Contemporary. I just loved the look of the room. While I would like to stay at the Poly or GF, I just didn't get that "warm fuzzy feeling" when looking at the pictures of the room.
Regardless of where you stay, hope you have a wonderful birthday celebration!