I'm guessing it depends on how it sounds when you modify the noun.
Wanter Vs. Wantster
the sufffix -ster is used when -er does not apply. -ster and -er indicate the same thing. -er as a suffix modifies a noun into something of an 'agent noun'
i.e.
a driver is the agent associated with driving
a player is the agent associated with playing
(Information found on Yahoo! Answers)
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