I have stayed at both POR twice and the Dolphin twice, and for your size family, I would definitely choose the Dolphin. We got the teacher's discount last year for $99/night and Disney could not beat that. But even for $129 for this year, that is still great. If my daughter (she's a teacher) were going, we would probably be staying there, but she's not and we got $94/night at POR. I just wish I could stay at the Dolphin this year as I'm going to miss the whole Epcot area. Don't get me wrong, I'm happy that we are going to Disney and feel very lucky, I'm just going to miss that area as it is my favorite. Also the Dolphin is a deluxe and POR is a moderate. They offer a lot more in the way of amenities.
To me the Epcot resorts have the best location and I feel the transportation is better. Besides being close to Epcot and MGM, the Boardwalk is right there and there is all kinds of activity going on in the evenings. I just love taking the water taxis to the 2 parks or you can walk if you like. I don't know if I would walk to MGM as it is a little further than Epcot, but I've heard that some do.
To me, the 2 don't compare, but this is JMHO. I know that some feel that POR has more theming, but the Dolphin has theming too, maybe not as much as Disney's resorts. It has an awesome lobby with a giant beach tent in the center. They have a great pool and game room. Also have a cafeteria-style food area....Tubbi's Buffeteria. They also have a 50's style soda shop and several table service restaurants between the 2 resorts. There are several restaurants nearby with the Y&BC and the BW.
Just wanted to give you my opinion. Again this is what I would choose. But you need to do what is best for you.
Either way, you can't go wrong. POR is a lovely resort with all the lush landscaping. I really like this resort also, but to me, it doesn't compare with a deluxe.
Good luck with your planning!