FbR is now a standart firing option for all mordern units, while KF needs to be researched and I would also say FbR is more powerful because u can use ur maximum firepower without increasing the size of ur formation as in KF.

While FbR is generally better in all siutations a good KF volley on a unit closing in for a charge can provide enough morale damage to rout or seriously waver them as well.