I would suggest going to Plymouth, myself due to my own personal experience of ARU. Biut it's up to you...
Cambridge is a good city, but sometimes as an ARU student you slightly get snubbed by the Cam uni students whilst out in town. Everything is close by, and in walking distance. And personally I feel quite safe walking around the city, even at night.
But ARU isn't the best. Depending on your lecturers, your classes can be good or bad. The I centre sometimes lose your work, but providing you have a reciept you don't have much trouble. The halls & SU are quite good, although on a Friday night, the 'Flirt' event can have the same repeated themes as the previous years (that's what I'm finding as a second year).
In my class, only five are mature students. Two of which, I guess, 40's and have children, the other are only a little older than me (I'm 22) So it really depends on which year you join, as people from foundation years would be joining you. But on the whole, the campus is quite diverse and many of them are the usual 18.
Have you been to both open days, and seen both campuses etc?