Maia wins easily. He's the better grappler by far and has been dominating grapplers at 170. Meanwhile Shields grappling has been completely underwhelming in the UFC and while he may be a good bjj guy he isn't ADCC champion caliber. Maia is one of the best in the world at what he does. Shields has not come close to proving that he is anywhere near that caliber.
That is not dead which can eternal lie and withs strange aeons even death may die.