I know there are lots of Dolphin fans out there but ... we have stayed at both. For the money we prefer PORiverside. The big plus for Dolphin is that it is in the Epcot area.
Since the Dolphin is a convention hotel, we found that the pool area was often closed for convention events in the evenings. You never really knew until you decided to swim. While you could walk over to the Swan or use the other pool -- it just wasn't the same...and it was irratating that they were closed. We also found that Tubbys and the other "eateries" at the Dolphin were VERY slow and expensive. Add a bunch of "conventioneers" who usually eat at the hotel and you couldn't get a bite to eat for lunch in less than 1 1/2 to 2 hours. It wasn't pleasant.
You might have a balcony at the Dolphin but we weren't in the room often enough for it to matter. The housekeeping while okay was not as "creative" as what we received at Riverside in November. Also, I think you have Queen beds at the Dolphin and Riverside only has doubles. But if you have two rooms then for the teenagers that won't be bad.
And, the transportation or the fact that you can walk to Epcot and MGM is nice. The boat ride, though, takes about 15-25 minutes depending on where you board so it is about the same amount of time as the transportation to anyplace else. And, at the Dolphin you share a bus with the YC/BC/Swan and Dolphin. That is only one stop shy of what you have at Riverside.
Advantages as we see them for Riverside. We prefer the food court there. You can get a fairly nice meal, in a relatively short amount of time. The variety is pretty good and while the prices are Disney, they aren't that bad. With teenagers, if you so desire, Pleasure Island is only a short distance for evening entertainment -- which they tend to enjoy. Shopping at DTD is also close and easy to get to in the evening, especially since they brought the boat back. It takes you about the same amount of time to get to any of the parks (15 minutes give or take unless it is very crowded). They have six quiet pools plus the main pool with a slide. It is never closed for conventions just for a "baby ruth" drill every now and then. (Happened when we were there in Nov. -- my 3 year old couldn't figure it out -- so we didn't explain).
So, in my humble opinion, if you don't spend that much time in your room (especially since you are getting two rooms) -- the fact that the Dolphin room is a bit bigger (by about 90 square feet - I think), and has a better location in proximity to Epcot/MGM, doesn't justify an extra $700. Since you've been to Riverside...if you liked it...I might spend the $700 someplace else.
On the other hand, if you want to try something new .... it might be the place. We just found that there was something "missing" since it wasn't really Disney (the Mickey soaps and shampoos in the bathroom and Mickey wake up call, for example). If the primary concern is money then definitely go with Riverside. If you want to try something new -- there are a lot of fans out there for the Dolphin -- I just prefer Riverside. Hope that helps.