It truly depends on what you wish to get out of your degree. If you're planning to study the Middle East, Asia and/or Africa in detail, SOAS is your best bet. UCL, on the other hand, has a tad more name recognition (in fact, the same goes for KCL). In addition, UCL and KCL are better for European studies.
With that being said, SOAS is a bit like the Georgetown SFS of London. Most of my friends in SOAS are people who are strongly opinionated and unafraid of voicing out their views. The institution does attract individuals who are interested in certain fields, so it's pretty specialised.