Coleman, 50, is standing as a figure of change. The Health Minister lacks the profile of the other candidates, though not the ambition. He is indicating that, should he win, then a shake-up of the cabinet is likely.
There is room in National's ranks for new blood. Besides Key, several other ministers are calling it a day, which gives whoever succeeds scope to present a rejuvenated lineup to the electorate next year.
In a ministerial sense, Coleman is the least experienced contender but his handling of the often troublesome health portfolio has avoided blaring headlines which can corrode political careers.
Collins is the most polarising, fascinating and perhaps riskiest candidate. She comes with baggage that would have wrecked the career of a less ambitious and resolute politician, which perhaps explains her apparent appeal to the wider party.
At 57, Collins is the oldest contender but does not lack for energy and drive.
Throughout her career she has embraced a take-no-prisoners style. Three years ago she resigned from the cabinet in a controversy over the Serious Fraud Office, but returned when an inquiry cleared her name.
Her pitch is simple - the party, she says, needs to ensure its appeal is as broad as possible, and connects with men and women and all ethnicities. It suggests that she is best suited to reach beyond National's traditional constituencies at a time when political connection is fragmented and unpredictable.
The end of Key's political career presents National with an unexpected headache and suddenly means that Labour, as the largest Opposition party, is back in the race. It means that the caucus leadership vote could have a pivotal influence on the outcome of next year's election.