Those leaders include European leaders such as Germany's Angela Merkel and French President Emmanuel Macron, as well as Canada's Justin Trudeau.
About 140 leaders are in New York for the annual leaders' week, and the New Zealand PM usually tries to catch up with those who will not be at other summits. This year Trump is one of those - he will not attend Apec or the East Asia Summit later in the year.
But when Ardern turns up to the formal dinner Trump is hosting for all the leaders Tuesday morning (NZ time), while she will not try to avoid Trump nor will she try to engineer a fulsome chat.
A photo of Key and Obama arm in arm was political gold back home, the more matey the better.
A photo of Ardern and Trump would be the opposite, especially among Ardern's base.
The next day Ardern will also listen to Trump speak at the UN General Assembly.
One of the focuses is expected to be Iran after Trump declared Iran had broken the anti nuclear deal brokered by his predecessor and imposed sanctions again, issuing a plea for other countries to follow suit.
It is yet another area of difference with other leaders. Ardern said she will continue to support the agreement. "Of course we were disappointed at the US withdrawal. Our view was that it had achieved what it set out to achieve."
Life was easier for Key. Obama was in favour of the Trans Pacific Partnership, the Paris Agreement on climate change, and had a "Pacific" tilt to his foreign policy to ensure US influence was not ceded to China. Trump has none of that.
Ardern has not bothered to hide her less than sympathetic view of Trump, even when speaking on his home soil.
In an interview on The Today Show in April, Ardern said she was "furious" to have been likened to Trump when it came to immigration - a comparison based on Labour's pledge to ban foreign house buyers and restrict foreign investment.
"The suggestion in any way that New Zealand was not an open, outward-facing country, they suggestion I was leading something that was counter to that value made me extremely angry."
The obvious implication is that Trump's USA was not open or outward looking.
But theirs was not a promising start. Prior to becoming Labour leader, Ardern had marched in the Women's March which doubled as an anti-Trump march after his election.
It was perhaps in retaliation that at their first meeting face to face at Trump reportedly mistook her for Trudeau's wife.
All might not be lost, however. Ardern's partner, Clarke Gayford, has been invited to a reception hosted by Melania Trump which might prove more productive in finding that common ground.