If scanning the QR code sends you to AppBrain, then it's because the QR code was generated to contain AppBrain's link. It wouldn't matter which app you used to scan the QR code, as they'll all send you to the same link.
However... the stock browser on my device will redirect me to the Android Market when it sees a link from AppBrain. I know for a fact that it didn't used to, so I don't know if it was an upgrade from the AppBrain app.