Both these books are accompanied by test-books so that you can check how well you learned your vocab, they are called 'Test your English Vocabulary in Use'. I think you should go to a bookshop, grab test books from both levels, do a few tests from each and this way you will see which one is more suitable for you. I would choose the level at which I scored about 40-60% because it would mean it is my level and I needed a to work a bit longer on it. If it is 80%, go higher. Hope it helps.
Interested in Language