First, I'm assuming you want to stay at one of the Premier on-site hotels for the EP option. If that is not accurate, then you might check out SF as it has a very nice pool, quality/service is similar to the Premier hotels, but since no EP, it normally is less expensive. Or possibly CBR, from a theme perspective, I probably like it better than any other on-site hotel. It is less expensive than even SF, but again no EP.
Second, traveling alone and deciding among the 3 Premier hotels, I'd ask what are your priorities? Cost first and foremost? If so, then go with the least expensive option. If "theming" is most important, does one interest you more than the other 2? Going solo, I'd probably not worry about the room size differences between the 3 hotels. While 450+ sqft vs. 335 sqft can mean a lot when you have 3, 4 or 5 persons in the room, when its only 1, it wouldn't be one of my big decision points. Going in January, you might not be planning on laying out by the pools as much as other seasons, but I will say that at I've found RPR seems to keep its jacuzzi's at a higher temp than PBR. Its been a long time since I last stayed at the HRH. So I can't compare it to the other 2. But I personally like to go sit in a nice fairly hot jacuzzi during the afternoons (when I'm traveling in January, and I will be at RPR this coming January.
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Also, does proximity to the parks mean much to you? HRH is about as close to US as a hotel could get. Yes, you have the water taxis and they are great, but if you like to walk back and forth to the parks, is location/proximity a factor? Note that I do believe if you plan to walk to IOA a bunch, RPR seems to be a slightly shorter walk than HRH, but none of the hotels are closer to US than HRH.