Aside from the fact that Cambridge has an offer that is easier to attain, you should take into account some other things, such as:
- Are you sure you want to do mechanical engineering, or would you like to keep your options open? (the first 2 years of the Cambridge engineering course are general and you can specialise - only if you want to - in the third year)
- Do you want to live in a huge city, or in a smaller one?
- How important is social life to you?
- What kind of scholarships would you qualify for at each uni? (even if finances aren't a problem, some extra funds never hurt)
In the end, both universities have great Engineering courses and a good reputation, so it's up to you to decide what else, besides the teaching, interests you when it comes to uni life. Also, didn't Cambridge make you sign a paper stating that if you were to receive an offer, you'd have to firm it? Or isn't that a thing anymore?