Either would be acceptable if the charge is equal to the oxidation state, but usually, you should use the #""^(2+)# notation for ion charge.
I would write #"He"^(2+)# to illustrate an actual charge on an ion, while I write something like #stackrel(color(blue)(+5))("N")stackrel(color(green)(-2))("O"_3^(-))# to depict an oxidation state.
This is saying that nitrogen has an oxidation state of #""^(+5)#, while each oxygen contributes an oxidation state of #""^(-2)# for a total of #""^(+5) + ""^(-6) = ""^(color(blue)(1-))# charge.
However, it doesn't say that nitrogen has an actual #""^(5+)# charge in the polyatomic ion.
It just says that if the bonds within #"NO"_3^(-)# were purely ionic, then nitrogen would have that charge. They aren't though (the electrons are evenly shared between the four atoms), so nitrogen doesn't really have that charge.
(The actual charge on nitrogen in #"NO"_3^(-)# is more like #""^(1+)#, while a #""^(2-)# charge is distributed evenly amongst the three oxygens.)
