Salisbury, definitely.

The cathedral is magnificent, the elderly volunteer guides are delightful, the copy of the Magna Carta with translation is thought-provoking, and if you can get in an evensong with either the boys' choir or the girls' choir, you'll marvel at what the children are capable of.

I enjoyed just walking around the city, too.

I also enjoyed Wells, which is not on a train line but is a short bus ride from Bath. It's the smallest of the cathedral cities, more of a small town, really, and quite charming. (If you saw the movie Hot Fuzz, that's where it was filmed.)