Amulet Heart is over rated and over used! And I'm somewhat of a Miki fan, so I prefer Amulet Spade.
Though, if you want to do a little gauging of ability a la gaming-wise...
Amulet Heart is more speed based... High mobility, is fast, and with quick attacks.
Amulet Spade is more attack based... Colorful Canvas has good area and stronger attacks.
Amulet Clover is more defense based... A healer. Nuff said.
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Amulet Dia seems support based... Though there's only one skill so far, it strikes me as a support type skill, soo...
Haven't seen a good example of Amulet Angel (or future Amulet things) so I can't really say what their basis is or whatever.