Which is better among the two? Leiden or Vienna for International Tax Law. How is the course Master of laws-Advanced studies in International tax law?
How does it differ from MA Taxation KCL?

I have the same question..

Leiden is one of the first (maybe the first) programs in International Taxation (I mean in time). Viena is newer but also a very good program (excellent).

Some faculties (professors) participates in each other (as visiting).

So, to decide I would say that you need to consider:

1- Price (and possible scholarship)
2- The subjects of interest of resident faculties, if you want one of them to supervise your master thesis
3- The city (cost of living, big, medium or smalle, boring, asier to find someone who speak English or TV or Radio stations in English).

As to the difference between an LLM and a MA. The first is a Master in Law (normally they only accept lawyers - people with a LLB but there are exceptions). A MA is a Master of Arts, its focus maybe more on the management-business-accounting or public policy perspective and they may accept graduates in business, public administration and political science, law, etc.

Hope this help

Leiden.