Krugman and Manikw are ideological adversaries. They both wrote Macroeconomics textbooks which fully agree with each other.

krugman and mankiw are adversaries in the sense one prefers the current world neoliberal order designed for the benefit of the us to be led by democrats and the other prefers it to be led by republicans

to the average citizen of the world they are indistinguishable

Subjectivism vs Labour theory of value is a pretty easy one.

Most of Econ 101, as a sibling comment notes, from basic approaches (definitions, deductive reasoning from models using algebra, testing of models with data) to detail (marginalism, "new consensus" macro modelling).

Macroeconomics and econometrics are two major subfields which literally didn't exist in Marx's day: economists may disagree significantly on conclusions and somewhat less significantly on certain assumptions models make, but they're focusing on the same variables (interest rates matter and fiscal policy somewhat matters, inflation is a thing which can be reduced, boosting employment and staving of recessions is more difficult) and doing similar maths.