As someone who is currently at uni (well, I’m on placement at the moment)..
You should have loads of time to ride.
Find out more before deciding-
What is the local riding like
Uni club – how good is it, and what/where do they ride
Do you have a car – if not, you will be limited to very local riding (or lifts from club/mates).
You’ll be a bit annoyed if you sell your DH bike, and then find out there’s some quality DH tracks on your doorstep with a keen group of uni riders.
Re: not taking a nice bike to uni, silly talk – while I was never in halls, loads of mates are and have had few problems with bike theft (one mate locked his up in a communal bike shed and it got nicked, go figure)