"I do not think that this idea is worse."
Translation:Nie myślę, że ten pomysł jest gorszy.
But that would be more like "In my opinion this idea is worse" – 'think' translates to "myśleć" so "nie myślę[…]" is better here, given how much Duo tries to keep the translations literal.
Outside of Duo, these are synonyms so there is no problem with exchanging one for the other. ;-)
Hmmm... I see that "idea" is among the translations of "pojęcie" on Wiktionary, but generally I'd say its main meaning is "notion". The only situation when I'd surely translate "pojęcie" to "idea" is "I have no idea" = "Nie mam pojęcia", but that's more of an idiomatic thing.