Ultimately its between prestige and employment prospects. If you care more about the long term prestige of a Cambridge degree, go there. If you are dead-set on a career on the financial industry and want a better chance of getting into IBD, go to LSE.
Not saying that Cambridge won't give you a great chance of getting an IBD job, but LSE absolutely dominates every other university in the UK when it comes to IBD recruiting. Although Cambridge gives you an advantage for MBB consulting.