On a much simpler level, not all win are equal. A battle where the last man standing is one of the PCs is technically a win, but I think that most players would regard that as a very bad outcome. You might want to break it up into four outcomes, instead of two: Side A wins intact, side A wins with casualties, side B wins with casualties, side B wins intact (maybe roll those last two together, if side A is assumed to be the PCs, because taking some down with you is small consolation for a TPK).