If you want to stay within Engineering once you graduate then I'd go with Imperial, if you're looking to move towards Management or potentially outside of Engineering then go to Oxford.
I studied Eng Sci at Oxford from 1999-2003, loved my time there and wouldn't swap but being honest I think Imperial has a better Engineering course although Oxford is certainly not a bad option
One thing Oxford does have over Imperial is a bit more flexibility in the course, the downside is you study quite broadly for the first year (and most of the second/third) so if you know you are only interested in Mech Eng then doing Chemical, Civil, Electrical and Information Engineering could be frustrating.
Oxford's night life wasn't (and I guess still isn't) as good as other universities but you can still have a great laugh as it your friends more than venues that make a good night out and there is nearly always something going on that you can be part of.
Make sure you visit a few of the colleges if you haven't done so already as they are quite different, I studied at St Catherine's which is more relaxed than some of the others (it's also ugly but it's what's on the inside that counts!) and I think I'd have found some of the more traditional colleges a bit suffocating.
What I will say is that getting work experience and showing leadership, initiative, etc outside of your course will be more important than whether you went to Imperial or Oxford because competition for the best graduate roles will be pretty strong. If I could tell myself anything in 1999, that would be it, I worked hard but not necessarily smart in terms of lining myself up for a job.
Disclaimer: I now work in IT in New Zealand, I wanted to stay in Engineering but life and opportunities led a different way.