I haven't read the whole thread so somone may have said this already but...
How big are the rooms at the Dolphin and how does that room size compare to POR? We are also a family of 5 but my boys are 7, 5 and 1. We typically stay deluxe but decided to go for a bonus trip just before Christmas (DH and I's gift to each other). We had already been in Feb where we stayed Atrium Club at the CR and in a deluxe concierge room at the BC so we blew our yearly vacation fund (plus extra from bonus $) on that trip. We had to keep the budget down for the December trip so we opted for POR. The resort was lovely and I have zero complaints about it. Beds comfy, beautify resort, I had no transportation issues, cast members were friendly, kids loved the pool...all of that. The BUT here would be that if our kids were 11, 11 and 4 we'd have been quite cramped. Not impossible but definitely tight. with only 2 kids it would be fine but 3 (when 2 are 11) it will be tight.
The Poly rooms are much bigger. I've stayed at the Poly on a "Mom and kids weekend getaway" with one of my girlfriends and her 2 kids + my 2 kids (at the time) and I. We had 2 adults, two 4 year olds and two 2 year olds in that room along with a crib and 2 double strollers and I didn't feel cramped. Granted still smaller kids but 6 people, a crib and 2 strollers. Is it worth the extra? Only you know that. You could get 2 rooms at POR for less than 1 at the Poly and have even more room and 2 bathrooms!
If you decide you are willing to spend the extra $ for a deluxe on the monorail consider the CR Garden Wing Deluxe room for $395. It is the size of 2 CR rooms so LOTS of extra space. It's similar to the BC Deluxe room we had in Feb and we LOVED the extra space. My kids actually had room to run around and play sword fight with the swords they insisted on buying outside POTC. Look at
http://www.disboards.com/showpost.php?p=17303966&postcount=4 for pics.
You won't go wrong either way though. I am a total deluxe girl and I was happy with POR and would return.