I don't know either area particularly well. Never been to Canterbury, but the cathedral looks impressive. I've been to a conference at Oxford university, and some of the buildings are impressive. The Cowbridge car plant & the John Radcliffe hospital are less so
Salisbury though ... nice little place. Best bit is the cathedral & specifically Old Sarum, just a couple of miles outside of town.
If you're heading Salisbury/Oxford way, and you're interested in ancient sites, I'd also consider places like Wayland's Smithy (neolithic chambered tomb), the White Horse, Avebury (including the obvious stone circle & henge, plus Silbury Hill & West Kennett long barrow), and various other spots located along The Ridgeway - http://www.nationaltrail.co.uk/ridgeway/downloads.asp?PageId=2