bxc3 gives you a majority of pawns in the center, and a big support for the outbreaking d4!, possibly pushing c4 and still having d4 pawn-supported. Big advantages on the long-term.
With dxc3, you're right, the bishop is free, but with already fewer pieces on the board, and Black being to play, it's not much of an advantage. The file for the Queen, “attacking” the ♟d7, which belongs on d6 where it is well defended, is worth nothing, and the possibility of castling queenside is not very relevant either (e.g. Black traded his f2- and h2-threatening bishop)
Also keep in mind that the c pawns are doubled for the time being, and you don't want this situation to last until the endgame. It will be a lot easier to trade the ♙c3 on the d file than trying to support the b pawn, or merely blocking the d file for Black.
1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6 4.Bxc6 dxc6, I always take with the d pawn, lots of good things can happen from here, also note that the pawn is poisoned, if he takes bring the Queen tod4– ajax333221 Jan 8 at 20:29