It's easier on the pocket if you do Dunkeswell and Old Sarum. I believe that this is a common Bournemouth one. Choice of Old Sarum might depend on which day of the week you go. i.e. w/e a bit easier.
Having said that, I did Exeter and Bristol and thoroughly enjoyed it, especially being marshalled in and picked up by a minibus at Bristol. In the week, Yeovilton will give you a service all the way round and, for me, they arranged hand-overs to Exeter and Bristol. Only Bournemouth would not co-operate with that one.
In my day, Exeter and Bristol had cheaper landing fees for students.