Hi there. Thought I'd chime in as I recently went through this same debate. For Club Carlson properties I was choosing between Mercer St, Plaza on the River, and the two Park Plaza's that you mentioned. I also threw out the Mayfair because of poor tripadvisor reviews. While I'm sure it could be fine, I'd rather stick with a top 125 hotel with 4.5 stars on TA which those four are. In the end I went with the Mercer St location because it is most centralized location allowing you to walk to loads of places and being near numerous tube stations. Being south of the river isn't as ideal IMO. I'm not using the Hilton points since I'm going to try and book something with them prior to the devaluation for another trip but felt that the locations for the best Hilton properties weren't as great.