Izuna's death was far from honorable (if he actually died), at the time the whole clan was disgusted with the act that they exiled madara from the village. To him the sacrifice of his eyes was meant to solidify the uchiha's power in the village, but the opposite happened instead it got his brother kicked out. It was here that I figure Izuna changed his opinion of the village and his clan to let such a thing to occur. A sacrifice of brothers for their people just to become subserviant to the senju's (brothers thoughts). Also if Izuna did in fact survive he would know that, the seal of the kyuubi weakened at child birth from living through the first hokage wifes fiasco. Also to me minato saying that when they fought brought back memories of madara (paraphrasing), just seemed to hint at this. This is why I figure it to be Izuna, just my opinion 