At level 60 a Shaman who has his spells (specifically Torpor) will heal sufficiently in almost any situation.  Torpor plus Regrowth plus Cannibalize means the Shaman can go longer than a Cleric can before running out of mana.  The Shaman also brings tools such as slow and malo which someone else has to provide if you have a Cleric. 
 
Of course Torpor is a very expensive level 60 spell which not all Shamans have.  Complete heal costs a few plat from a vendor at level 39.  You can be sure the Cleric will have his most important healing tools while you'll have to ask a Shaman in advance. 
 
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