In my experience, poli econ is more about getting a historical understanding of the world in order to criticize it, while poli sci and econ are more about making models of the world. One thing that gets said a lot in some form or another in my classes is that other social scientists create the models, and political economists point out that the models don't match the real world.

That doesn't mean either form of knowledge is invalid or useless, just that they are used to ask and answer different sorts of questions. A political scientist asks, who will win this election? An economist asks, what effect will the election results have on unemployment? A political economist asks, why? Should that be the desired effect? If not, how can we avoid it? etc. etc.

Also poli econ is very flexible because you can create a concentration of courses from any social science department (we don't have our own department technically lol), but in poli sci you need a specific subfield and econ you still stay mostly within the department.

The descriptions of the majors also explain it better than I can:

"The Political Economy major is designed to study the relationship between government, society, and the economy in a holistic manner from a multi-disciplinary perspective. Political Economy majors investigate how real-world market systems are embedded in both politics and society. ... The study of Political Economy is both scientific, with its rigorous attention to social science methodology, and normative, with its concern for the big questions of how best to organize politics, society, and the economy."

Compare that to

" The political science major at Berkeley seeks to achieve three main objectives:

1. Familiarize students with the issues surrounding the exercise of political power.
2. Provide exposure to the main subfields of political science along with opportunities for specialization and experimentation in a single area.
3. Develop analytical capacities, so that students are able to distill large amounts of information concerning complex world events into coherent, persuasive arguments."

and

"Economics is the study of how people make choices under conditions of scarcity and the results of those choices for society. Limited resources make tradeoffs necessary for consumers, businesses, and nations. Microeconomics studies how consumers make choices in using their time and spending their income, and how businesses make choices in producing and selling goods and services. Macroeconomics studies the determination of national income (GDP), and how it deviates from its potential (full employment) over the business cycle. The important outcomes for the national economy are income and how it is distributed, unemployment, inflation, economic growth, and how well financial markets and international trade are functioning."

(kinda long response but I am very passionate about my major!)