Castors for under stairs storage unit
Part of the answer to this will be determined if you are pushing in the unit from the side of the stairs or from the taller space into the lower height wedge space. Fixed casters may be more appropriate for the side access because there will be benefit to the guided straight in/out positioning where the storage unit will be higher on one side than the other.

It is common to put fixed casters on one end of a moveable item and the swivel casters on the other end. The best advantage is having the swivel casters on the end you push/pull from so that you can steer the cabinet into its final location. It may help to envision a grocery cart that is usually equipped with fixed + swivel wheels. Sometimes you just have to get ahold of the front of a grocery cart to back it up into a tight spot.