by the numbers in 2018 Oxford took 18% of engineering applicants, Cambridge 15% so not a significant difference for either. really this will vary person to person: are you better at the ENGAA (Cambridge) or the PAT (Oxford)? try some past papers to see which you prefer. Cambridge also interview a higher percentage than Oxford so if you think you're stronger at interviews than tests you may want to consider that.
however, really you should be choosing the uni with the course you prefer, as both are similarly competitive so make sure you are applying for something that is right for you