I was going to say it was a tough choice, but then I thought about it more. I'd go Swan in a heartbeat. In the end, it really depends on what is most important to you.
PO-Riverside. What you get at PO-R that you won't get at the Swan is Disney themeing in a self contained resort. PO-R is very nice. From check-in, to walking around the quiet paths, seeing the Mansions and the Bayous (two different sections of buildings within POR-R), the central area with the restaurant/food court, the mill and water wheel, the little bridge over the water that leads to the pool area - it is all very charming. IMHO, PO-R is one of the best themed of the Disney moderate resorts. If that type of themeing is important to you then PO-R is a great choice.
Swan. There are a couple of things to consider with this resort. First we can compare themeing. The Swan and Dolphin hotels themselves are not themed as much as PO-R. Many people say that the Swan and Dolphin have more of a 'hotel' feel than a themed resort feel. That is somewhat true. However, the Swan and the Dolphin are surrounded by the YC/BC, BW, MGM, and Epcot. They are right in the middle of one of the most active areas of WDW. So, what the hotels themselves lack in themeing, they make up for by being right in the middle of so much other themed Disney stuff. The location is simply incredible. Other things to consider at the Swan - the pools are nicer, the rooms are bigger, you have easy walking access to more restaurants and entertainment, you can walk or boat to both Epcot and MGM. So if location and all these other things is more important than having a Disney themed resort, the Swan is the place.
We have stayed at both. We really like PO-R. However, all things being considered we would stay at the Swan if we had to choose between the two. If we could get the Swan for the same price as PO-R it would be a no-brainer.