What if any difference will be noticed in a Brushed XL5 Stampede if changed from using 8.4V Nimh to 7.4V LiPo?

Wouldn't more power be provided by the Nimh since it is 1V higher than the LiPo?

For equivelant MAH in batteries (e.g. 7.4V 4200Mah & 8.4V 4200Mah) will one last longer than the other or give about the same run time? What advantages would be achieved from the LiPo?

Thanks for your help,