I’d say that Das Kapital is more useful. The manifesto was a specific call to arms for a specific time. It’s worth understanding as a historical note, but, it’s not really a theoretical work.

Das Kapital, is a work about a theory of political economy and layed out the actual basis of Marxist theory.

Everything else is sort of down stream of that. At least in regards to understanding what specific objections or concerns people in Marxist thought have.