Hi, I actually live in Saint John! I am very familiar with both cities. Saint John is a wonderful, walkable city, beautiful waterfront, great local shopping and beautiful brownstones and parks. Lots of great restaurants and some amazing coffee shops. I really recommend Catapult Coffee & Studio as everything is baked on site, locally roasted beans, and handmade items. We have the famous city market, one of the oldest in Canada. Princess Street, Germain Street, King Street, Market Slip, the Loyalist Buriel Ground, all are lovely walks and close to the cruise port. I know that there is the pink double decker hop on hop off buses running and you can also take day trips to areas close by and in some cases, the trip can be an hour away.
Halifax is Saint John on steroids. Much more populated, more shopping, more restaurants, etc. Both are hilly cities, and many beautiful tourist areas nearby. Let me know if there was somewhere in particular you were interested in.