I asked some of my colleagues this question and the general consensus I got was that CFA was a broader qualification, generally more respected in the industry and would suffice for an excellent CV.
However, that is not to say that CISI does not have a useful purpose or cannot compete with the CFA. What my colleagues told me is that CISI does very well in certain niche areas and they're an excellent resource for development and CPD. In particular, they say some exams can be harder than CFA (they all cited the Derivatives paper, shortly to be withdrawn and replaced with a capital markets paper) and very respected in their niche. Also, they are of course UK centric and cover local financial regulation in more depth than the CFA will.
So I would say get your broad base first and then when you know more specifically where your long term career is going, then give CISI a look.