I never noticed before, but there's a good big chunk of the Outlands -- the 5th and 6th rings -- in which mages can't cast plane shift, but clerics can. This just became tangential to a plot point in Spire's Shadow, and it struck me as interesting.
There ought to be a useful ramification of that. The first thing that strikes me is that you could have a monastery or other religious refuge for a cleric-led group that was on the run from a wizardly organization with a long reach. They would have an avenue of ingress and egress not available to their hunters. More offensively, the clerics could use it as a sortie point against a wizardly org: their location would allow them to planejump, strike, and retreat, while preventing their foes from doing the same thing to them.
Here are the spells from the SRD that are different levels for clerics and Sor/Wiz, not counting domain spells, or bard/druid/ranger/paladin spells. I may also have missed a few on my skimthrough.
Clerics can cast where wizards can't:
Hold Person Clr 2, Sor/Wiz 3
Gentle Repose Clr 2, Sor/Wiz 3
Animate Dead Clr 3, Sor/Wiz 4
Bestow Curse Clr 3, Sor/Wiz 4
Remove Curse Clr 3, Sor/Wiz 4
Contagion Clr 3, Sor/Wiz 4
Control Water Clr 4, Sor/Wiz 6
Dismissal Clr 4, Sor/Wiz 5
Sending Clr 4, Sor/Wiz 5
Plane Shift Clr 5, Sor/Wiz 7
True Seeing Clr 5, Sor/Wiz 6
Banishment Clr 6, Sor/Wiz 7
Refuge Clr 7, Sor/Wiz 9
Of the spells listed, dismissal, plane shift, and banishment look like the most relevant to a Planescape game. A band of sea or ice clerics with a settlement in the 5th ring would have access to dismissal and sending for defense and coordination, and control water as a tactical spell, while arcane foes would not. A settlement of necromancer priests (death or undeath, to preferred flavor) in the fourth ring would have access to a significant number of spells in their repertoire that arcane foes would not.
Mages can cast where clerics can't:
Blindness/Deafness Clr 3, Sor/Wiz 2
Continual Flame Clr 3, Sor/Wiz 2
Locate Object Clr 3, Sor/Wiz 2
Obscure Object Clr 3, Sor/Wiz 2
Magic Weapon, Greater Clr 4, Sor/Wiz 3
Tongues Clr 4, Sor/Wiz 3
Scrying Clr 5, Sor/Wiz 4
Antimagic Field Clr 8, Sor/Wiz 6
Repulsion Clr 7, Sor/Wiz 6
Not as much of interest that I can see here. If a group of mages wanted to hide something where clerics would be less able to find it but mages could, the 3rd ring of the Outlands might be promising.