The differences are minimal, cranborne is definitely a very clean, modern, spacious and very nice halls but doesnt have that layer of polish dorchester does like big windows, leather sofas, double beds, shelving etc etc. Cranborne feels like a nice hotel whereas dorchester feels quite apartmenty. Cranborne is literally next door to purbeck (which might be being refurbished next year mind) and chesil, as well as not being far from dorchester and again pretty close to lyme regis. Also that bit nearer to the clubs/takeaways as well.
If you prefer the luxury aspect you wouldnt be missing out on going out as Dorchester will have its own crowds roundabout the peak clubbing times, heading to and from, as well as the fact its pretty big so youll meet a good amount of people who live there while out and about/in uni. The only difference is that being outside of the lansdowne triangle of halls, you wont experience as much ambient chaos on nights out outside the bunch of halls together, and its a little bit further (5-10 mins) from town/halls. Both halls have bus stops and co-ops right outside and are equally distanced from asda/the gym.
A thing to consider though is the fact dorchester constantly gets oversubscribed due to it being the best halls on paper, which ends up with a lot of people getting put in unilet rather than other halls. So i picked cranborne to be safe, and im glad i did mostly due to neighbouring areas/distances in general. Plus im biased now as ive grown to love my new home!