Within the parameters you have specified, both boats are about equal...
If there is any sort of possible "red flag" at all, it might be that the old Volvo is less desirable than the old Universal diesel. Parts for the Volvo, when or if needed, will be more expensive in general. Against that, the Volvo's are known to be quite reliable.
Both boats are average sailors in lighter conditions, but the Islander gets the nod for sailing potential with a moderate beam. The Cat is equally flagged for being almost a foot wider which equates to superior "gracious dockside living" but less forgiveness if forcing a hull through a three foot chop.
Given their age, you will want to find out if and when either has had standing rigging replaced. If either still has the old wire-to-rope halyards, that is another small but right-away expense. Check to see if the mainsheet tackle and traveler has been upgraded to at least a Garhauer on either boat.
Mind you, nothing wrong with ancient systems, but after three decades a lot of stuff, from sailing gear to plumbing to wiring... is in the twilight of its useful life.