My two cents worth...
1. For law, my understanding is that Paris II has the best reputation (regardless of overall reputations). People in the legal field with some familiarity with French universities might share that opinion (...or they might not).
2. In which jurisdiction do you want to practise? If you would like to qualify as a British lawyer (even one then hired to work in the French office of a British firm), do you need the double degree? Or do you see yourself qualifying as a lawyer in France (I've heard it's harder to get a job in law there)? Have you checked out the qualification procedures? (In my opinion, law is a profession which it is very difficult to practise in a second language, given the importance of nuances and the precise use of language. You may disagree.)
3. Similarly, would you be better off studying 3 years of UK law and 1 year of French law, or 2 years of each? To what extent would you be limited to core subjects in each jurisdiction or would you get to do optional subjects that might interest you and help you get a job? (I don't know the answer, but do pay attention to which subjects you will get to do in each course.) Of course if you plan to do a Masters after, that could allow you to do optional subjects, but do think about it.
4. Are you bilingual? Will you struggle to compete on an equal footing with French students in a top French university? I presume from your offers that your French is excellent for a British student, but is that the same as being able to cope and get good marks in a French Law School? A single year abroad would be less challenging, especially as visiting students generally get more freedom in choosing subjects. Check out what if any preparation (eg language, legal methodologies - very different- and introductions to basic principles) each course offers before going abroad and what workload and required course you are required to take in the Paris II year abroad. Also, how are your final grades worked out?
I think my main points are: what are your long term plans? and look at the detailed structure of both courses.
PS For more information, if it's not on the internet, you could trying e-mailing/phoning up (it might work) or looking at the Paris I website.