Both are great actors but I'm not sure it would have been quite the same.
Pacino gave such a convincing portrayal at the beginning of GF1 of an innocent young man not trying to get in the business, etc. etc. and his emotional changes as he is drawn into the film are one of its most moving themes, at least to me. I'm not sure DeNiro could pull that one off, without any disrespect to DeNiro.
Secondly, DENiro's portrayal of young Vito in GF2 is simply briliant, a fantastic job. He adopts Brando's mannerisms and speech in a flawless manner. It would have been difficult for Pacino to portray a young Vito, but not impossible, I suppose.
That is a good question!
