A good way of comparing this is by comparing voting records. partijgedrag.nl has a comparison matrix in which you select one party and compare similarity with other parties on a large number of categories. When selecting the Socialist Party, you immediately see which party is the most similar overall. As you may have guessed, it is the PvdD.
Since the matrix breaks votes down by category, we can actually go into investigate a bit further in which categories the most difference occurs. There are 17 categories in total and the similarity per category ranges from 80% to 92%. Unfortunately, I cannot link to the categories individually as they don't have dedicated URLs. The way it works is that you click on the percentages in the first column and then it shows the votes in that category. The votes seem to be fairly recent, ranging from 2009 until 2018.
The following categories are the most similar (90+%):
Migratie en Integratie (migration and integration): 92%, Internationaal (international): 91%, Onderwijs en wetenschap (education and science): 91%, Recht (law): 90%, Zorg en gezondheid (healthcare and health): 91%
Then slightly less (though still very) similar (85-89%):
Openbare orde en veiligheid (public order and safety): 88%, Sociale zekerheid (social security): 87%, Huisvesting (housing): 87%, Bestuur (governance): 86%, Economie (economy) 86%, Financiën (finance): 86%, Landbouw (agriculture): 86%
Then the categories on which the parties are the least (but still very) similar (80-85%):
Cultuur en recreatie (culture and recreation): 84%, Werk (labor): 84%, Natuur en milieu (nature and the environment): 83%, Ruimte en infrastructuur (public space and infrastructure): 83%, Verkeer (transport): 80%.
So especially those last categories are where the two parties differentiate themselves. Most notably I think are the transport and infrastructure categories.