Well, I cannot help you in terms of having any personal experience with these institutions and courses. On the other hand however, I have accepted my offer to attend UCL's MSc in Security Studies, and will be there this September.
I got other offers for Durham's MSc in Defence, Development and Diplomacy, as well as Kings College London's MA in Intelligence and International Security (which is part of there famed War Studies department).
Basically, I accepted UCL's offer as firstly, the course approaches the empirical study of security in a broader sense, focusing on the human aspects of security as well as the hard military forms. Additionally, the quantitative and qualitative core modules seem to offer highly employable skills that other courses such as at Kings simply don't. Kings in contrast, is very much more concerned with the military aspects of security studies, something which I felt made it too niche in terms of potential career prospects. Secondly, the School of Public Policy (SPP), which the course falls under, is the only political science department in the country to focus on postgraduate studies. This means that you are the main part of the department's functions, and not some sort of on the side job/ cash cow. Or at least, this is how I percieve it. The SPP does have two undergraduate courses, but they are not huge in comparison, whereas there are 400 postgraduates on the masters programmes. This roughly equates to around 50 students on each of the 8 masters courses, although the open day made it seem like the public policy courses are the most populated, so there are probably between 40-50 students on the Security Studies course. The SPP buildings are marvellous, and are based on Tavistock Square and Gordon Square. The department seems highly geared towards research and further study in the form of phd students. Moreover, the SPP at UCL includes its highly influential Constitution Unit.
UCL was ranked 4th in the world in the QS 2009 and 2012 world ranking, 17th in the 2012-2013 Times Higher Education world ranking, and is consistently amongst the top ten in the national league tables. Equally, from 1999-2009 UCL was the 13th most cited university in the world. I felt that having a university brand name like UCL on my CV would be more advantageous (as it feels as if it has a greater international standing, whereas a university like St Andrews is less so unless your American of course), and basically had the edge over KCL and Durham (which are both very highly respected universities nevertheless). UCL's undergraduate politics courses seem highly respected with league tables stating it had around 80% for graduate prospects, so I would of thought this success would filter off into its postgraduate courses. UCL also has the Yale UCL Collaborative, which enables Yale and UCL phd students to do exchanges at each others respective institution, which is surely testament to UCL's academic prowess. The Sunday Times called UCL ''an intellectual powerhouse with a world-class reputation'.
This may not help, you probably know all of this if you have done good research on the department already anyway.
Good luck with your decision!!