Disagree with the other poster, RHUL would probably impress me more than City or Reading, also it's absolutely beautiful. I don't really know people who went to any of these unis though so idk.
In terms of avg graduate starting salary for economics it's City>RHUL>Reading
In terms of avg UCAS points of students for Economics it goes RHUL>City>Reading
Location wise City offers the biggest difference from the other two being in London. Can't really comment beyond that, though.