In my opinion Cass is the best one year MBA program in the UK. The rankings might prove otherwise but bear in mind that FT/Economist rankings have a 3 year lag and so if a business school makes changes, the rankings take more than 3 years to reflect that.
Anyway, to answer your question - actually, to not answer your question but to add some content to the argument that academic life at Cass is exciting, why don't you read my post here on the Cass MBA blog.
Of course if you want to read other posts and just stay in touch with what we say on this blog, just bookmark A Cass MBA blog
I am not very familiar with Lancaster, but I believe that Cass is a better brand than Lancaster, particularly if you want to work in Finance. If you look at this chart LINK you will see that recruiters rank Cass higher. As you mentioned, Cass has the London advantage and offers very good career opportunities. Both programs are ranked among the top 12 in Forbes rankings. In FT rankings, Lancaster is placed better, but the methodology used by FT is questionable and not really relied upon internationally (Harvard 5th? CEIBS 11th before schools like Kellogg which is only 24th?? Plus 2008 rankings are based on data from the classes of 2004, 2003 and 2002!), . Be aware that I applied to Cass, so my opinion is somewhat biased. In any case, good luck with your future decisions :).
I have an admit from Lancaster and Cass for the full time MBA program. Please let me know which one would be a better option.
Things going for Cass
1. Located in London and the job oppurtunities are better
Things going for Lancaster
1. Cost when compared to Cass
2. Better brand and reputation
Job prospects is the main criteria.