tamzCongratulations on getting the admit!
I don't have a first hand experience of the course or the place, so I am going to share what I have learnt from different sources.
Vallendar, as a place, is not much appealing to students. So in that aspect, Cass is obviously a good choice given that it's in London. But honestly, this aspect shouldn't matter much. You'll not live in Vallendar for 2 years. You'll secure an internship (which means you'll leave Vallendar) and you'll leave the place for your exchange semester as well. So, all in all, you get to stay in Vallendar for not even a year. But yes, London is the capital of the world in quite a few aspects.
Ranking definitely puts WHU above Cass. Furthermore, Cass is not the best in England whereas WHU is a leading business school in Germany. You'll get to learn a new culture and a new language, which is a big advantage. Ofcourse, if you're looking to work in Germany post your masters, you must be proficient in the language. I haven't had any contact with Cass alumni but I have always heard good things about WHU. I did not apply to Cass because it is not a top school in Europe as per MIM rankings.
I am not really informed about your future goals. So, from what i know, i would suggest you to go for WHU over Cass. It'll be a much more challenging experience, in my opinion.
Let me know if you have more queries.
Cheers