I would always take the piece rather than the exchange. The Bishop might be useful to the opponent and you have to think more in the endgame when you are the exchange up.
R4B or N4 free
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I would have taken the Knight, simply because it gives me more material. Also, I don't need to give away any of my attacking forces for his, and I get a free prisoner! In addition, a Bishop is worth more than a Rook in the Middlegame, so in your case you were actually losing material by trading like that.
In a recent game I had the opportunity to gain a rook for a bishop, or to gain a Knight without risk. It was in the middle game with both of us having most of our other pieces, including queens and rooks.
I wasn't sure what to do - in the end I swapped the bishop for the rook as the position was soon to open up and I figured the rook would be more useful in the end game.
Do you think that was the correct decision? I'm still not sure.