Marriott London County Hall is the 120th on tripadvisor’s ranking. It is a 5* Hotel by all means. I have stayed there, at the JW Marriott Grosvenor House and at Marriott Grosvenor Sq. County Hall with JW are the best and I believe that are the best Marriott’s in town. Junior Suite at this property will give you probably a Big Ben view and executive lounge access which are great benefits.Also the County has excellent spa/swimming pool.
On your question now, I don’t mind (assuming that the small room is not a hole) as far as the hotel has some standards and Marriott properties in London have those. I would choose based on my location’s needs.